In Belize, book a tour from the ship or on your own. This is not a port you want to venture out without a plan. The ship's excursions are very crowded for this reason. There are several independant operators who provide a nice tour in Belize. A couple of them are mentioned in this thread. We personally did the Coral Breeze tour and we echo the comments above....the best we did. It was a highlight of our week.
Good luck and have fun.