I here that snorkeling and scuba diving are the best things to do in Cozumel. They have a Mexican nation park there, I think it is called "Chanaab" (or similar) where the snorkeling and underwater reefs are the best, or so I heard.
In addition to that, the shopping is really good too. If you drink, I hear that the liquor prices are really good. If you are into baking, the vanilla they sell sure beats the stuff you find in your local supermarket. Mexico has abundant silver deposits. So, they have allot of things made of silver, for sale in Cozumel. Make sure if you buy something made of silver, that it has the 0.925% purity standard (or similar numbers). There is allot of interesting souvenirs to buy. My favorite shop for souvenirs, when I was in Cozumel, was "Cinco Soles". That means 5 Suns in English. Both the merchandise and the people working there were nice.
You could also hang-out with the party crowd at Carlos and Charlies. I heard wild things go on there (not when I was there, unfortunately). Maybe I should return around Spring Break time
You could even go upstairs above C&C's to Seņor Frogs. I never saw two party places together (one on top of the other) before. Lets not forget about the Hard Rock Cafe, too; somewhere else in Cozumel.
So, I hope you have a good time on your cruise and also while in Cozumel