Based on a 6 month stay in Geneva and the Lac Leman region in 2003/2004.
Housing
Don´t be discouraged by "la crise du logement", you´ll find something eventually. My favourite neighbourhood is Plainpalais, followed by La Jonction and the Old Town, possibly also Paquis or Les Grottes.
The most common housing option for young people are foyers (similar to student residences), particularly if you plan on staying in the city for less than a year. They are often booked, since many of Geneva´s students are also staying there.
Check out the university´s housing webpage
for short-term sublets and for more information on the various foyers in the city (see also www.unige.ch/cite-uni/logements/accueil.html). Beware, most of them are Christian which can be a nuisance in many ways (some are girls-only, there may be restrictions on having guests overnight, and you might face some missionary attempts...). However, Foyer George Williams is a perfect example of an open-minded Christian foyer (best proof: condom machines in the corridor). Not a bad place to stay, pretty good location, reasonable rent (at least for Geneva standards). If you are in the city, you may want to pick up a brochure called "Logement pour jeunes a Geneve" (available at e.g. Foyer George Williams). The tourist office also has a webpage on housing for young people and their brochure Info jeunes might be useful for short-term housing during the summer. Last but not least, the black boards at the university are always a good resource for shared flats.
Communications
Telephone
Concerning mobile phones, I found that the best deal was getting a Swiss sim card (Sunrise pronto) at Telecartes Specialistes, near the main bus stop in La Jonction (where I got the actual sim card for only 10 CHF).
For local calls from payphones, Taxcard phonecards are the best option. All payphones also take incoming calls.
Internet
There are computers with free Internet access in all the university buildings. For more sophisticated computer work, there are many Internet cafes in the city (for example an overprized one at the train station, or one at Boulevard Carl Vogt).
Money
Withdrawing sufficient amounts of money from cash machines can be somewhat tricky as the maximum amount you can get seems to depend on a variety of factors (the particular bank in Geneva, your home bank, your type of card etc.). I found that the cash machines at La Poste worked best (1000 CHF at a time). You can also pay for a lot of things with your credit card.
Food
While the main Swiss supermarkets Migros and Coop are definitely high quality, they are also very overprized. Groceries and pretty much everything else can be found at much lower prizes just across the border in France. Depending on where you live in the city, you should head for either Ferney-Voltaire or St Julien. Hyperchampion is a huge French supermarket chain that sells mostly groceries, but also electric appliances, clothes etc. (Walmart-style). The home brand "Champion" is usually reasonably prized and "Le moins cher" (with the 1 symbol) the cheapest. Groceries at Lidl may be cheaper that at Hyperchampion, but they have a pretty small selection of products, so it´s less fun to shop there.
Cheap options for "eating out" (budget-style) are Migros restaurants, university cafeterias (though those can be surprisingly pricey) and, most of all, Manora: This restaurant, which is part of the Manora department store, has a wide selection of cheap and high quality food and is also open in the evenings and outside normal shopping hours.
Libraries
You can get a card for all public libraries in Geneva for free if you can proof that you are a Geneva resident (the receipt for last month´s rent at your foyer etc. is sufficient). The public library in the old town also has a collection of audiotapes. In addition, the university libraries are generally open to the public (though you will probably not be able to borrow books).
Sports
You could try to sneak into the university sports program as in some places they don´t check for student IDs. There is also an international students club at the university that organises skiing and other activities. You can rent bikes for free for a day at different locations (e.g. behind the train station). For more information on cycling in Geneva, have a look at www.geneveroule.ch.
Movies
Geneva really has a great movie scene, including many artsy movies in various languages. There are also frequent international movie festivals. To find out about movies and movie theatres in the city, check the movie website. Movie theatres I particularly recommend are Cinelux at La Jonction and Les Scalas in Eaux Vives. Maison des Arts du Gruetli often hosts international movie festivals. It also has an affiliated theatre with interesting plays (tickets only 10 CHF for students). Alternative movies are shown at Spoutnik. For free movies, check the university´s film program and Bistrok (part of the Rhino squat in Plainpalais).
Bars
Alhambar in the old town is a great place. Plainpalais also has tons of fun, relaxed student bars (for example along Rue de l`Ecole de Medecine and Boulevard Carl Vogt). Cafe Cuba at Place du Cirque is also recommended. For more alternative bars, check Artamis and l`Usine or Bistrok (part of the Rhino squat in Plainpalais). Carouge also has some nice cafes/bars, I heard good stuff about Le Chat Noir. Clubbing is usually pretty expensive, with the exception of l´Usine which regularly hosts all-style dance nights for 8 CHFs. For cheap or free concerts, take a look at www.akouphene.org and La Galerie (Rue de l'Industrie). I also had good reports about Cafe les 5 portes (a bar in Rue Zurich) and Comme a la Maison (a cafe at Rue ancienne 36).
While Geneva is a nice city, its surroundings are unbeatable. I spent every weekend touring the Lac Leman region, the Jura and the Alpes. It´s best to have a car for this, as trains and buses in Switzerland are usually quite expensive and their routes and schedules often not particularly helpful.
Don´t forget to get a good map, e.g. the Kümmerley-Frey map of Switzerland at 1:275000 (available at FNAC or Librairie Payot in Geneva). www.mapquest.com is also useful. Weatherforecasts for Switzerland and France can be found at www.meteosuisse.ch and www.meteofrance.com.
If you don´t have a car, check the Swiss train website for cheap "Click and Rail" offers. You can also get a daypass that is valid on all trains in Switzerland (44 CHF) at www.freiefahrt.ch. For train and skipass specials, take a look at www.railaway.ch. To get to St Cergue in the Jura, take a regular train to Nyon and then a minitrain to St Cergue. For regional buses, try the Gare routiere website (including good skiing specials combining skipass and transport). For long-distance coaches to other countries, check out Eurolines (tickets available at Autotourise Leman, Rue du Mont-Blanc 14) and Gullivers. Regarding rental cars, I heard that Autohire in Ferney-Voltaire is not bad. General information on backpacking in Switzerland can be found at www.swissbackpackers.com.
Hiking
By far the best hiking guide is Daniel Anker´s "Hiking around Lac Leman" (or something along those lines) from Bergverlag Rother which is available at e.g. Hyperchampion. There is also an English language guide called "The Happy Wanderer" available at the Elmworld English bookstore. If you are a keen hiker, but don´t have a car, you might consider joining a hiking club like Rando-74: They offer organized hikes with carpools from Geneva for a small fee.
Good hikes are La Dole, Mont Biere and Mont Tendre in the Jura; Le Mole, Cornettes des Bises and Grammont on the southern side of the lake; and Mont Buet in the Alpes.
Skiing
Brevent-Flegere in Chamonix (France) was our favourite (weather forecast for this area: http://meteo.chamonix.com). Avoriaz and Contamines are in the same general area and also good. Verbier in Switzerland is very good, but further away. The glacier in Diablerets is ridiculously flat, but you do get some great views.
Other cities
You should at least make it to Lausanne which is a very nice city with a different feel from Geneva (more students and normal people as opposed to bankers and diplomats, large pedestrian zone, located on a hill with good views of Lac Leman). Annecy in France is also highly recommended: very scenic location at beautiful Lac Annecy, historic town with many small lanes and canals. You might also consider going to Bern to visit the historical old town and the national parliament and to get a taste of German-speaking Switzerland.