Based on a one week trip to Croatia in 2007.
Croatia seemed like a very interesting country and would love to come back. Beautiful Meditarranean scenery, amazingly clean water, great food (particularly seafood) and lots of nice islands. As far as cities are concerned, Dubrovnic is supposed to be fantastic. We stopped for a couple of hours in Zagreb which had some nice pedestrian areas and was pleasantly untouristy. Among the many islands, Rab Island and Losinj Island have been particularly recommended to me. Rovinj on the Istrian Peninsula is supposed to be very pretty and a good place for diving.
Avoid the main tourist season, particularly August, when the island gets just incredibly crowded. Also, consider heading for nearby Losinj island which is supposed to be more off the beaten track. It's a good idea to have a car and explore the island. There are lots of nice Mediterranean oak forests and nice hills for hiking (particularly in the south towards Baska). Make sure you don't forget your snorkeling gear (and shoes to protect you from sea urchins). Beware that there are no sandy beaches, but the great snorkeling opportunities along the rocky shore will definitely make up for it.
Useful links:
Krk Tourist Info
Autotrans (buses Rijeka-Krk)
Lonely Planet Croatia
BUG Croatia