Think of this as Puerto Rico's version of the Galápagos. Mona Island, 46 miles west of Cabo Rojo, is open to just a few hundred visitors at a time, with the only overnight stays at campsites along its white-sand beach. Inland, nearly 90 percent of the terrain is 200-foot cliffs, riddled with half-mile-deep caverns. These contain the skeletons of many a conquistador and pirate. (Legend has it Captain Kidd once stayed here.) But come for the wildlife. On a four-day trip with mainland-based Acampa Adventure Tours, let four-foot Mona iguanas come to you like poodles, chase after blue-footed boobies, and spy on feral boars (you can also hunt them with bow and arrow in winter). Acampa arranges pickups throughout Puerto Rico, and trips should be booked one month in advance (roughly $750 per person for groups of ten; acampapr.com).