Ive been to the cozumel area 3 times which is very close to Cancun and I highly recommend you go to Xcaret. Its an eco-archealogical park and it has sooooo much to do. The have an underground river system that is very unique as it is saltwater/brackish and you can snorkle it(we did and it was fun). They also have mayan ruins, a beach area with more snorkleing, dolphin encounter, aquarium, mayan cultural shows, zoo, butterfly exhibit/pavillian.... just a whole lot of stuff and the place is gorgeous. Its a great way to spend an entire day and its inexpensive. Here is a ling to thier website:
http://www.xcaret.com/