Saba, Caribbean Netherlands
A steep volcanic peak that juts from the sea, Saba is a 15-minute plane ride from St. Maarten. It's tiny — five square miles — but tops out at a misty and mossy 2,850 feet. The island's sheer cliffs mean that you won't find many beaches, but this won't seem like a problem once you get in the water. "The diving is spectacular," says Brent Winner, a Florida-based marine biologist. Sharks, rays, and turtles swim among coral-encrusted volcanic pinnacles like Diamond Rock. (Book dives through Sea Saba, which also organizes snorkel outings.) "Afterward," says Winner, "you can climb the trails that crisscross the island and see from 2,000 feet up some of the formations you just dived. I've never experienced anything like it." Another option: Just take in the view from the Queen's Gardens, a luxe resort clinging to a cliff 1,200 feet up.
Getting there: From St. Maarten, take either the 15-minute flight or a 60-minute ferry ride.