Been to Cozumel several times and several different months of the year. This is a favorite port for us and there is so much to do even if you don't book an excursion. If you go to Carnival's website, you can look at all the excursions available in Cozumel and book ahead, or you can wait until you board to book.
If you don't want to book an excursion, you can do what we do. We alternate between Paradise beach, and Chankanaub State park in the morning, then head into town to shop and hit Margaritaville for a little while.
Been there - done that:
2005 Holiday, Sensation, Conquest,
2006 Conquest, Celebration, Holiday,
2007 Freedom Grand Med, Holiday
2008 Fantasy & Sensation,
2009 Fantasy, Holiday & Dream Grand Med
2010 Fantasy and B2B Elation
2011 Monarch of the Seas
2012 Booked - Breeze from Barcelona 12 days