Cozumel and Belize have some of the best diving and snorkeling you will find anywhere. I would suggest doing a snorkel excursion in Cozumel and if you dive then do that in Belize.
The cave tubing has been a popular excursion in Belize but if the water is high it can be a little more excitement than you may want.
In Costa Maya I strongly suggest doing a Mayan ruins excursion. It will be well worth it. Costa Maya itself is merely a shopping mall. You can go to the village of Mahahual, just a short cab ride away, and enjoy the beach, restaurants and local bars and shopping.
Enjoy yourself and I know you will enjoy the cruise.