dpel28,
last time we went we spent the day at Chankanaab. It was ok, See link for reviews.
http://www.tripadvisor.com/Attraction_Review-g150809-d185578-Reviews-Chankanaab_National_Park-Cozumel_Yucatan_Peninsula.html
This year we're spending the day at Nachi Cocom.
http://cozumelcruiseexcursions.com/CozumelBeachBreak.htm
The other times we were there we did the party/snorkel boat and shopping.