In Belize, one of the biggest selling shore excursions, at least from what we hear, is the cave tubing. Many like to do the zip lining with it, but the cave tubing is extremely popular. If you're more into sightseeing, there are some amazing Mayan ruins in the area.
And in Roatan, it's zip lining followed by snorkeling or scuba diving at West Bay Beach. It is the reported to be the 2nd largest reef system in the world and because it very close to the surface in many areas, it provides one of the best opportunities for snorkeling anywhere on the planet! It is absolutely amazing and if you have ever wanted to snorkel, this is the place. Scuba diving there is awesome!