A valid passport (US Citizens do not require a Visa).
Current drivers license (if driving).
Major credit cards (Visa is best) are accepted nearly everywhere. Colones (Costa Rica currency) can be exchanged at the current rate and US Dollars are accepted everywhere as well.
A big hat, sun block, insect repellent, flip flops, comfortable shoes, sunglasses and a camera are a good idea. In the area is always warm so bring light clothes like shorts, tshirts, dresses, skirts and bikinis.
There are no vaccinations or medical concerns and the water is safe to drink inside the resort.
If you are taking a jungle or rain forest tour you might want to bring long pants and rain gear.