The FATW site has a bit about Cayo in it, and they loved the cave tubing: "This is a unique opportunity to go on a guided tour through some of the areas impressive caves – where you’ll see ancient Mayan artifacts that date as far back as 400 AD as well as the amazing cave formations and crystals that adorn the ceilings and walls of these alien-looking caves – all from the comfort of an intertube floating down a brisk subterranean river."