boilermaker- make time to go to Isla Mujeres. It is a short ferry ride from Cancun, but the scenery is great

, and one of the coolest things I have ever done, was swimming with the dolphins.
Watched people do it at Xel Ha(sp?), and was not impressed, but at Isla Mujeres, it was worth every penny.
I am sure that your hotel will have a tour desk, and I would ask them about a trip to Isla.
If you have not been to Cozumel, I would not waste a day to see it. We rented a Jeep and drove around the whole island, not much there to see.
As rrc mentioned Casa Rolandi is a nice place to eat.
Enjoy yourselves, as there is plenty to do.