La Isla de Guilligan (Gilligan's Island, though not the one from the television show) is a popular hangout for Puerto Rican locals. I myself am only home for several weeks a year and I still go once or twice every year.
The island is sometimes isolated and sometimes crowded. It boasts snorkeling (I've spotted many anemones, fish, crabs, starfish, and barracudas), calm water perfect for floating and swimming, and several trees from which you can jump into the water.
It's off the coast of Guanica, and there are two ways to get there. Either take a ferry (call 787-821-5706 for more information) or rent a kayak from Mary Lee's by the Sea. If you choose to do the latter, you might want to take your kayak to Isla Ballena (see my highlight "Kayaking and Snorkeling and Hoping for Whales").
This tends to be a day trip, so I suggest you have some food. You can either pack up your own or, if you take the ferry, place an order before embarking and tell them when you'd like to have it delivered. There are picnic pavilions that you can claim if it's a calm day or you get there early.