Jet Skis. Let of fun. Safe, easy, inexpensive. BUT BE CAREFUL. The number one rip off in Thailand is renting Jet Skis. Only rent from rentals recommended from your major hotel chain. Here's the rip off. You rent a jet ski from what seems to be a nice beach guy, but when you return it, the bottom will be all scratched up and they will blame it on you and try to overcharge you thousands of Baht ($100's). While you know this is a scam, the police, when called will side with the locals. And you NEVER want the police to be called! SO before you rent ANYTHING, inspect the item 100% even if it means to turn it upside down. Take photos! That way you can avoid a long and unpleasant situation.
Most tourist activities have a recommended set price. These prices are usually posted near any of the major hotels or Tourist Police stations. When in doubt, ASK! Assuming nothing. Always negotiate your price before you agree to rent any item. Thailand is not very regulated with these matters and when a dispute occurs, the local police will ALWAYS side with the Vendors.