Definately take a day and go Snorkelling in Cozumel it is like swimming in an aquarium amazing.
Babes is one of our favourite places to eat also El Fogen on 30th and 28th for a real Mexican experience as this is a favourtie spot of the local. There are just to many wonderful restraunts to pick from.
Playa Info has lots of wonderful tours to take look under the directory. Heather will help you out.
Parasailing is another fun thing to do. Hope you have a great holiday......