I've been to all 7 continents and Costa Rica is in my top 4 favorites. If you like eco-tourism or adventure travel, this is a great place.
Try to get to Tortugaro National park on the East coast. You do little boat "safaris" where you go by boat through these canals and watch for wildlife. Everything from caimans to monkeys to lizards to toucans to you name it. Awesome experience. We actually got to witness turtle hatchlings busting their butts to get to the ocean one night. One ran across my hand as it was pitch black and you had to put your face inches from the sand to actually see them. Another got held up on a piece of drift wood until I intervened and moved the wood for him to keep going. AWESOME, AWESOME experience.
In the middle is the continental divide which has some crazy rain forest canopy zip line tours. Be sure to take them in.
On the west coast is where we did our R&R at an all inclusive which was also fun.
The only downside is that it takes forever to drive anywhere. The roads are absolutely terrible - some of the worst i've ever seen including the 3rd world. If you can spend a little extra $$$ to fly, do it, because an extra $50 might actually save you 15 hours or so, which is an extra day to do something.
If you want some cool souvenirs that are not nick nacks, then check out Galeria Namu in San Jose. We heard about it in the Lonely Planet book and it was everything that it promised. It is the closest thing you'll get to "Fair Trade" artisan products. check out their website at
www.galerianamu.com or check out the write up in the Lonely Planet. I liked it so much that I've kept the business card in my wallet since we were there, which was November of 2005! Anyone who goes to Costa Rica I point them in that direction.
good luck.