The most challenging part of a trip to Saba, a five-square-mile volcanic island 28 miles southwest of St. Maarten, is the arrival. Saba has the shortest commercial runway—1,312 feet—in the world. Think of it as an aircraft carrier made of rainforest and cliffs. But once you touch down, your toughest decision is whether to hike into a cloudforest or dive among coral-covered seamounts. Base yourself in a hot-tub-equipped cottage at Dutch marine biologist Tom van't Hof's Ecolodge Rendez-Vous (doubles, $85; ecolodge-saba.com). Then hike past sweeping ocean vistas on the way up 2,877-foot Mount Scenery. Post-hike, head for the centrally located village of Windwardside, home to Sea Saba Advanced Dive Center. The outfitter leads half-day snorkeling trips and four-day scuba-certification courses in Saba National Marine Park, which van't Hof founded 20 years ago (snorkeling trips, $35; dive course, $450; seasaba.com). Winair flies daily from St. Maarten to Saba (from $150; fly-winair.com).