Based on a 5 month trip in 1999.
Costa Rica is an absolutely beautiful country. First of all, it is a great place for nature lovers with amazing rainforests and beautiful tropical beaches. Costa Ricans (or ticos as they like to call themselves) are very warm and fun people (particularly if you speak Spanish), and experiencing the Latin American lifestyle for a while is the best possible cure to western workaholism. Last but not least, it is a very safe country with comparatively high social and health standards and an amazing health care system (a lot of people from the U.S. actually go to Costa Rica for treatment). As a result of good health care combined with low levels of stress, life expectancy in Costa Rica is higher than in many Western countries.
Democracy and pacifism have a long tradition is Costa Rica. The country has been a democracy since the 19th century. It had a predominantly peaceful history (interrupted by just one civil war that lasted only a few days!) and has no army today, a fact that Costa Ricans are exceedingly proud of. No wonder Costa Rica became a destination of choice for the Quaker movement when the Quakers had to leave the U.S. because of their pacifist attitude. Today, Costa Rica is a major holiday destination for U.S. Americans or gringos (as they are derogatorily called by Costa Ricans). Costa Ricans dislike U.S. Americans, but they really like Europeans, so if you happen to be from Europe, be sure to let them know...They absolutely adore Spain, but people from other European countries will also be greeted with enthusiasm. Costa Ricans will be more than happy to teach you that America is actually a continent (rather than a country), so the proper term for people from northern part of America is "norte americanos" (or just "gringos" :-)).
As in most Latin American countries, family is very important, so don't be surprised if one of the first questions you are asked by strangers (right after your name) is how many brothers or sisters and children you have (and people in the countryside may be somewhat surprised if you are in your early twenties and still don't have kids). By contrast, that obnoxious "So what do you do?" question that Westerners like to ask in the same situation would be considered pretty meaningless by many Costa Ricans. Obviously, everybody is "doing" something, like help the neighbour repair his roof, or care for an elderly parent... Religion (catholicism) also plays a very important role which is reflected in many commonly used expressions (like "que dios te accompana"). Costa Ricans also do believe in good manners and some formal politeness. But most of all, they are very warm, friendly and laid back and love to tell jokes, have fun and dance and party. A typical answer to how you are doing is "pura vida" or "suave", reflecting the general attitude. In a study on values in different countries, a vast majority of Costa Ricans said they believed that "enjoying life is a moral value".
Like many Latin Americans, Costa Ricans have a very "interesting" concept of time...If you ask for something and the answer is "manana", don't take this literally. "Manana" is best translated as "not today" and could mean anything from days or weeks to never. Likewise, if you are asking when the bus will finally arrive, a very popular answer is "ahorita". Again, this should not be literally translated to "any moment", it could still take several hours. Efficiency is not exactly something that Costa Ricans are striving for. Two very common answers you will get when asking for something is "no hay" (we don't have it) and "no sirve" (it's not working). And you will quickly get the feeling that you are probably the only person who thinks that something should be done about it. If you politely ask when somebody will come to repair whatever is not working, the answer will invariably be "manana"...
Costa Rica is an ideal country for the individual traveler and is also very safe for women traveling alone. There is no need to prebook any expensive holiday packages. The tourist infrastructure in Costa Rica is great, with an extraordinary bus system and many small family-run hotels and restaurants. Compared to Western countries, food and accomodation are really cheap (a few dollars for a great meal or for a night in some basic cabinas). Basic hotels usually don't have hot water, but in that kind of climate, you may find a cold shower more refreshing anyway...
By far the most important thing to consider is the season you are traveling in. Climatewise, the dry season in the winter months is of course the best time to travel. You probably won't have much fun in the rainy season during the summer months, as torrential rain for days or weeks could strongly limit any outdoor activities. However, for obvious reasons, the dry season is also the main tourist season. December to March are probably the busiest months. During that time, crowds of tourists in every national park or preserve might seriously spoil any wilderness experience. The best thing to do is choose one of the transitional periods for your trip. For example, traveling in the spring (e.g. in April or May) might be a good idea, as the main tourist season will be over, but the rainy season often won't have started yet. In the year that I was there, the onset of the rainy season was delayed by many months due to El Nino, allowing me to enjoy an uncrowded yet mostly dry Costa Rica. Note that even in the dry season, days often start with a clear sky, with clouds gathering during the day followed by a major thunderstorm in the late afternoon. So make sure you start your day early (around 6 a.m.), so you have had a full day until about 4 or 5 p.m. when the torrential rain starts. Then it is time to curl up in a hammok and chill until it gets dark around 6 or 7 p.m..
Most Costa Ricans do speak some English, particularly if they run hotels or restaurants. However, if you really want to meet people and experience the culture, speaking Spanish is key. Costa Rican Spanish is very similar to European Spanish which is why many consider Costa Rican language schools a good place to learn Spanish. There are some major differences to European Spanish, but they usually make Costa Rican Spanish easier to learn and understand than European Spanish (see the use of "usted"). Most importantly, Costa Ricans speak very slowly, and they absolutely don't care if you make plenty of mistakes (as they feel that they also don't speak Spanish as well as the much admired Spaniards). One typical thing about Costa Rican Spanish is that people like to add "ito" or "ita" to many words, e.g. chicito, or even chicitito instead of chico. There are also many terms that are different in Costa Rican and European Spanish (e.g. forest is "bosque" in Costa Rica, but "silva" in Spain). Educated Costa Ricans will still understand you if you use the Spanish terms, but to people in the countryside, you may sound a little funny...If German happens to be your native language, there is one great way to learn the basics of Costa Rican Spanish in no time (even if you have no previous knowledge of Spanish): the Kauderwelsch language guide for Costa Rica. When asking for directions, keep in mind that Costa Ricans are very eager to please, so rather than admitting that they don't know, they will often just make something up...best to ask several people if you want to be sure.
Last but not least: Do bring a very good mosquito net (surprisingly enough, I had a hard time finding a place that sells them in Costa Rica). It is also a good idea to bring something against diarrhea. Immodium should only be used as a last resort, e.g. if you get really ill on a several day hike. A good drug to help your stomach settle into the new environment (with its different E. Coli strains) is Tannacomb. It has no side effects and can be taken as a precaution on your first days in the country or whenever your stomach feels a bit weird afterwards.
For your orientation, download a map of Costa Rica.
Monteverde: The Monteverde Cloud Forest Preserve is very beautiful, with good hiking trails and different types of forest (depending on the elevation). The preserve was originally founded by the Quakers. Maybe you will come across on old guy with a machete on one of your hikes: that's Wolf, one of the original founders and an absolutely great person. The locals affectionately call him "il loco", because he basically lives in the forest and is always seen walking around, day and night. Avoid visiting Monteverde during the the main tourist season, as the preserve can get very crowded. And make sure that you do the overnight (2-3 days) hike to El Valle, Aleman and Eladios. Maybe you could even ask if Wolf has time for a guided hike to some other less accessible areas which would be a great experience (I got to go with him because I was a volunteer, so I'm not sure if he does guided hikes for tourists). Also, ask if there is any space at La Casona, the main shelter in the preserve. Usually, only staff and volunteers stay there, but they sometimes make exceptions. This will allow you to stay in the preserve overnight and you will have the nearby trails all to yourself when the preserve closes in the late afternoon. Plus the food at La Casona is very good. Take the opportunity to go on one of the many guided walks that are offered at Monteverde, particularly one of the night walks. If you like doing manual work, particularly trail maintenance work, you might consider volunteering at Monteverde.
Corcovado: Corcovado National Park is absolutely amazing, a true tropical rainforest wilderness combined with beautiful beaches (though not suitable for swimming due to currents and various dangerous animals..). Make sure you do the several day hike to La Sirena, a famous research station (try to stay there more than one night as there is plenty to explore in the area). If you feel like a little bit of luxury, chill out at the remote Marenco Lodge for a couple of days.
Manuel Antonio: Manuel Antonio National Park is very easy to reach from the capital San Jose. Its beaches are absolutely gorgeous, with beautiful trees providing shade. Be prepared to share the beach with some friendly iguanas...Don't forget to walk through the forest behind the beach where, with some patience and luck, you can see monkeys and sloths.
Gandoca Manzanillo: Don't forget to spend some time at the Caribbean Coast and enjoy the tropical beach paradise atmosphere with many coconut palm trees and the distinct culture of the people living in this part of Costa Rica. The Gandoca Manzanillo Wildlife Refuge is very remote but well worth the trip as it is major nesting site for the world's largest seaturtles (la tortuga Baula). To see the mother turtles lay eggs, watch the hatching tortugitas and help protect the site from poachers, consider volunteering at Gandoca Manzanillo for a week or so (after a week, you will probably be a bit worn out by the frequent night patrols on the beach, the heat and particulary the really obnoxious sandflies...).
You should also take time to explore Costa Rica's great volcanoes. The active volcano El Arenal is the most famous, but Rincon de la Vieja National Park is also well worth a visit, with its many geothermal hotsprings and an amazing waterhole complete with a waterfall. There are numorous other really beautiful protected areas in Costa Rica. For an overview, visit the website of the Costa Rican national park service. There is also a volunteer in parks program run by the nongovernmental organisation ASVO. However, I didn't get a positive impression of their program as they seem to keep most of the registration fee for themselves while the parks only get a very minor fraction. So you are basically supporting the administration of this organisation rather than the national parks. I wonder if it might be possible to negotiate a volunteer stay with a particular park directly...
A note for hard core budget travelers: While food and accomodation in Costa Rica are pretty cheap, entrance fees for national parks can be quite hefty (often around 10 dollars per day). If you spend most of your time in national parks, this could really strain your budget and you might find yourself in a situation where you almost spend more money on entrance fees than on food and lodging. I was lucky to find a very nice way around that. The high entrance fees only apply to foreign tourists (even if they are students, so no point flashing your ISIC card), while Costa Ricans only pay a symbolic fee of less than a dollar. In my first month in Costa Rica, I took a one week language course at one of the schools in San Jose and got a letter from the school saying that "la senorita Schmitz is a student at our school from the month of". As a result, for the rest of my trip, I was basically regarded as a Costa Rican student of foreign origin and therefore ended up paying the fee for Costa Ricans...
I can't wait to go back to Costa Rica for another trip. I'm looking forward to spending more time at Corcovado National Park and also have a look at Tortuguero National Park which I didn't get to visit on my last trip due to an outbreak of dengue fever in the area.