Here's Kevin's list of things to bring with you to Costa Rica:

1. A flashlight - and don't get a cheap one
2. Spare batteries for the flashlight
3. A compass if you are going to be driving anywhere
4. Sun tan lotion - a 30 or better
5. Duct tape - and figure on leaving the roll as a gift for someone
6. Some black tape if you're going to be doing any electrical work
7. If you bring a camera, don't bring the high tech model: suntan lotion and sand will usually put it down. And bring 12 exposure rolls of film (different speeds for different light) because that way you will still have some pictures after your camera is stolen from your tent
8. Some spices if you are going to be doing your own cooking
9. And learn some Spanish first