rainforests= monteverde (HORRIBLE road......now some say its worth it) otherwise stick to the aerial tram in braulio carrillo road, its a 5 hour tour or so
volcanoes = irazu,poas and arenal, first two are 5 hour tour from San Jose, for the other you better go to La Fortuna and spend a few days there, Venado Caverns are cool if you like adventure
Also Turrialba volcano, if you want to do some real exercise you can try to get to the top from Turrialba Lodge which is a hotel right in front of it, its an awesome view but this volcano in particular is not active right now
I love Puerto Viejo and Manzanillo, that's in Carribean side, its not as freaking busy as Guanacaste and its closer to San Jose as well, its also very cheap as the accomodations are not even 3 stars (don't expect a tv or phone in the room)
december/january and feb probably not the best times to visit as everything is swamped with both locals and tourists