A swimsuit and a good towel. You have read the Hitchkikers guide haven't you?
A good pair of walking shoes and sandals.
Snorkel gear. We rent some out but it is always better if it fits you like it should.
A flashlight. Neccessary for walking at night.
Acidopholis pills. Good to take anyway. Start taking them a week before you leave.
Sunglasses.
A good book or three.
Citronela bug juice. You never know. Bugs own the world.