Cave tubing is a must...stick with the guides on the way to the river and learn an wealth of information about the flora, fauna, customs, remedies and Mayan culture. The people of Belize are proud and so friendly not to mention the food is excellent.
Drive south from the port and visit El Cedral the locals are fun here. We headed straight across the highway to Albertos for lunch. Head back out on the highway eastbound to see the most beautiful beached ever. On the way back throuhg the middle of the island make a right to San Gervasio, it is a must see on Cozumel.