A valid passport and a photocopy of your passport. Costa Rican visa, if required. (Visas are not required for citizens from the US, Canada, and most Western European countries. Check with a travel agent ahead of time to make sure.)
No vaccinations are required. Check with your local health department for recommendations.
Health and travel insurance
Backpack or weekend bag
Insect repellent & sunscreen lotion
Camera and lots of film (expensive here)
Sports clothing; lots of shirts/t-shirts as you will want a daily change of clothes in the tropics.
Beach towel & swim wear
Umbrella and lightweight raincoat or poncho
Pen and paper or notebook
Prescription medications; spare glasses or contact lenses
Small alarm clock
Waterproof moneybelt or belly belt
Some pocket money (USD in small denominations) for the first few days. Travelers checks in US dollars. (Don’t bring other currencies.)
Dictionary (English-Spanish/Spanish-English dictionaries are for sale at ILISA)