14. Grab one of your most adventurous friends, rent a pair of Jet-Skis for an afternoon, and go hit the water for a few hours.
Unless you've got a boat license, you can't legally rent a boat and take it out on the water. But you can secure a Jet-Ski (in most states!) for a few hours by taking a simple course, learning how to ride, and then taking your new water ride out for a spin. It's a great way to stay active once the beach gets boring.