my wife & i were in costa rica 11/13-21........we had a pretty rough go of it, but i think it was just a really bad run of luck. the first 3 days were really rainy, but we were in a beautiful spot (
www.waterfallgardens.com/lapaz-peacelodge.htm) so it was nice anyway. then i got some sort of fever that knocked me out for two days, and then we had a 6.2 earthquake!!
all that aside, it is a gorgeous country with very friendly people. every hotel we stayed (3 in all) had well organized excursions every day, sometimes involving other nearby hotels, so you would never have to do anything alone if you didn't want to.
they are just coming out of the rainy season, so the weather is on your side for the next 5-6 months.
i can tell you more if you are interested.......