In response to Nick8's post (which is a great summation - I agree especially about how friendly and comfortable the country and its people are), I found it much more relaxing to take Interbus around the country than drive. Costa Rica's public transportation is very good, especially if you want to get around locally. Moving longer distances is a bit time-consuming on public transit, but Interbus and Nature Air will move you around the country quite nicely.
If you really want to drive, go for it. If you're nervous about driving in another country, or just want to hang up the keys for a while, Costa Rica offers some great options.
Also, there's a jewel of a town on the Nicoya peninsula called Montezuma. It's full of locals, hippies, backpackers, and in-the-know Europeans. It's a bit rougher-around the edges (not dangerous, but you'll see people in tents on the beach and some litter here and there) than places like Manuel Antonio or Jaco, but there's just something about it, an energy there that I've missed since I left. The whole country is full of hidden gems, but that one lingers in my mind.
One other thing Nick8 said that really resonates with me is that there are thousands of amazing places in the world to see, but I would go back to Costa Rica again at a moment's notice.