Well, it depends on what you want to accomplish. The neat thing about the place is that there is a nice range of environments to enjoy. There are rainforests, beaches, volcanoes... do you like adventure stuff? Nature stuff? Do you want to relax at a spa or tan at the beach? Do you want to get stupid drunk in a big cheap house without having any run-ins with the locals? Do you want to dance and soak in the pura vida spirit with the locals?
I went a few years ago, so I have a few recommendations depending on what you're looking for.
