Depending on what your into CR can vary tremendously from region to region. If you're into jungle, remoteness amazing wildlife, nice beaches the Osa Peninsula is the way to go, I like to stay at
http://www.bosquedelcabo.com/home/index.html, the food is great, the service is great, my favorite place really (it does help that there are three world class waves there and Pavones is a short boat trip away too
).
For a less involved laid back trip
http://www.manuelantonio.com/ is a cool spot, you can visit old growth forest and turn around and have a nice dinner. There's a few towns close that you can visit, Quepos, Jaco, Dominical (south of Domincal is stunning), so day trips are the plan in MA (my g/fs folks have a house at the top of the hill in MA, and it's where we decompress before and after the adventures and swap boards [I have three there
] and do ding repair).
IME the best time to go is the beginning of May or the end of November. The green season (December-April) is the prime tourist time down there so everything is at a premium, if you go outside of those months things tend to be much cheaper, and less crowded. That said be prepared for rain, it is a jungle.
In the North I haven't done too much exploring, just surfing in the Nosara area so I can't comment on it other than it had less of a variety of plants and wildlife.
There's no real threat of violence down there, petty theft mostly, so if you're traveling in a rental car, take precautions. I always travel in a group, so that has never been a problem for me and my friends.
Drink tons of coffee and bring some back (cCafe Milagro if you can get it), put Lizano on everything you eat, and eat tons of seafood for the full experience.