Do the Cozumel Dune Buggy Tours. It was a great time when I went to Cozumel last. It's an all day long deal with a tour guide, your own car, you get to go around the island, you snorkel, sight see, show, they feed you lunch and you get all the beer you can drink.
Awesome tour. I booked trhough an agent but you can also e mail firstname.lastname@example.org