I was backpacking all over Costa Rica last August and would highly recommend it. The pacific side has many more resorts and development while the Caribbean side is more laid back and, to be honest, different from the rest of the country. I recommend the following areas:
1) Manuel Antonio - very touristy and busy BUT the beaches are incredible and there are tons of monkeys (in addition to the ape variety
) around.
2) Monteverde - It's way up in the mountains but the cloud forest is a must see in my opinion. Also, go on a canopy tour somewhere.
3) Cahuita - Super laid back Caribbean town on the SE of Costa Rica. The main street looks similar to the one someone posted of Montezuma. On one side of town is a black sand beach and the other sports a pristine white sand one (Playa Negra and Playa Blanca). There is also amazing snorkeling here. I would recommend not renting gear from the guy with leprosy though.
Another place that I didn't have time to get to myself but have heard good things about is Bocas del Toro in Panama. Check it out on google, it is a group of islands on the NE part of Panama (not too far from Cahuita).
I'm not a resort kind of guy, so I don't have any input in that regard.