We had a great time on Isla Providencia, a mountaintop poking out of the sea far from anywhere else. It's full of pirate history and the name "Morgan" features prominently in local landmarks. The island, which is part of Columbia, is friendly, clean and safe, and the trade winds keep the temperature comfortable. We liked it so much that we stayed twice as long as planned.
Diving and snorkeling are the main attractions, but the island is sailboat-friendly too. There's free wireless on the plaza by the dinghy dock, and you can rent mopeds to tour the whole island inexpensively. The book exchange is at the coffee shop between Panadería Sunflower and Coffee Red Internet.