I lived there in 2001, and went back in 2003 for a time. It's absolute paradise.
Places to go:
Manuel Antonio National Park - you can chill on the beach w/ monkeys.
Dominical - Surf beach, not very crowded.
Quepos - Cool town.
Jaco - Surf town - overrated.
Downtown San Jose - Lots of tourist shops / restaurants.
Volcan Poas - Very cool volcano, and right where the earthquake was.
Volcan Arenal - Active volcano, you can usually see lava at night.
Limon - Caribbean Coast - aka Jamaica. Awesome food & beaches - hot as hell.
Tortuguero - Swamp / jungle guides. Hard to get out there.
Playas del Coco - Awesome tourist town up by Nicaragua.
Flamingo - Tamarindo - Panama - Linda - Nosara - Samara - All world class beaches. YOU MUST CHECK THEM OUT.
Liberia - Town at the crossroads of the northwest coast. Cool place to chill.
Anyway, that's just to get started. If you want more info than that, let me know.
Make sure you keep your passports on you at all times. The authorities love North Americans, but sometimes like to "check you out". Change most of your dollars to Colones - everything is cheaper in Colones ... and you'll get "took" if you pay in dollars.
Take cameras and batteries!!!! Those are double the cost down there.
Lots of car rental places... but the roads suck, and they are crazy drivers, so drive defensively.