1. Belize City, Belize
Bring a book, some jams (and maybe a cocktail) for the bus ride from Belize City to the Crystal Cave, where you'll start by ziplining through the forest. Then tube through the sacred underworld caves of the ancient Maya. Led by a guide, you'll traverse a pitch-black recess with nothing but a helmet light to illuminate crystal-covered stalagmites and stalactites. At the end of the tubing route, swim in a cool, deep underground lagoon before returning to the sunlit surface.
Shore excursion $159.