Night tours x 10 !
www.facebook.com/Nighttours1/
Cahuita national park (east coast)
Absolutely without a doubt the best part of Costa Rica. We saw more wildlife (not just tarantulas) than any other place combined.
It actually warped our impressions from then on, as noone came close to Chris. We did about three other night tours after and they were just "oh, did you hear that" or "there's a moth"... Chris actually spotted a different amazing insect, tarantula, bat, stick insect etc literally every few steps.
People will disagree, but I'd actually avoid La Fortuna. It's so so over-hyped and waaay too touristy and expensive. If white water rafting is your thing, go... otherwise it's a tourist trap of £75 coffee or chocolate tours which are basically walking around someone's garden looking with a few chocolate plants, expensive gift shops and rusty machinery, jungle rope bridges for £50, which are just 25 m bridges over the rainforest looking down onto clouds, and poison dart tours for the same price (maybe) seeing one or two frogs, otherwise looking at piles of leaves.
I'd rent a car... MUST be 4x4, and search for all the national parks and waterfalls, and just visit them all.
..and go see Refugio Nacional de Vida Silvestre Gandoca Manzanillo... it's a trek to get to, but trust me, this was our number two! Go on a hike into the jungle past the beach ;).
Also, friendly word of advice... stock up on snacks in any supermarket or corner shop you find. You get ridiculously bored of beans and rice after a few days... literally served at every meal!