Gabbles1, I was in Cozumel a few years ago, not on a cruise however. We spent a week on the Riviera Maya and one day in Cozumel. The shopping there is fun, a lot of silver jewelry at great prices. I don't know what excursions your ship offers, but I spent the morning shopping, ate lunch at Hard Rock Cafe and went snorkeling in the afternoon. It was a nice day.
I don't know if this information will help you since I took these excursion from our resort, but I'll include them anyway:
One of the excursions our resort offered was a trip to the ruins, Chichen Itza. Really neat place to see but it was a long ride and very hot the day we went. We stopped at a sinkhole to snorkle afterwards which was great!
The highlight of our week however was an excursion to Xcaret, their version of a water park. We swam in underground water through caves, saw many exotic birds, beautiful beaches and watched a show put on by costumed people on horseback. Xcaret was a hit with my 15yr old daughter. I would gladly give you more specific information on Xcaret if you are interested.
Whatever you choose to do, have a great time.