Hi kstopps, welcome to Cruise-Chat! Congratulations on your up-coming cruise! Please check out our Caribbean Port Chat section a little further down the main page, it has posts about all of your ports. You can use your "find" feature in the upper left hand corner and type in your ports, it will pull up all the posts made about them. Without knowing what y'all like to do, it's a little difficult to suggest activities.
You can take the supershuttle, cab, limo, or the cruise lines transportation. Cab fare will be about $75, which is for the cab, not per person. SuperShuttle around $25-30. Limo $75+, cruise line around $40/pp.
We're doing a similar itinerary on the Epic next week. Instead of Belize (which I love), we're doing Roatan, Costa Maya and Cozumel. We're snorkeling in all of our ports.
Hope you have a marvelous time!