Concierge.com's insider take:

Belize has an enviable endowment of navigable caves. Some of the best are just outside the capital city of Belmopan, along the aptly named Caves Branch River. Others include the so-called Actun Tunichil Muknal cave (a Mayan reliquary where you'll see pottery, skulls, and crystallized skeletons), and the caves at Jaguar Paw—where a guided float through the darkness and silence is the inner tube–borne equivalent of taking tranquilizers. But ensure in advance that you're not visiting this last complex on a cruise-ship day, when not only the local caves but also the usually serene rain forest zip lines swarm with furloughed passengers. For more information on caving, go to www.travelbelize.org/cave_index.html.