In Cozumel we went to Chankanaab Park for the day. We went swimming with the sea lions and did the helmet dive. Had a great time doing both. The park offers beach chairs, shade huts, restaurants, bars, restrooms, lockers, and showers. They also have a botanical garden and a replica of the Mayan Ruins. The kids did some snorkeling right off the beach. They brought some dry cereal with them to attract the fish.
In Costa Maya we walked around the shops at the port and then rented a golf cart and drove to Mahahual. Its a small, rustic village about 2 miles from the port. They have a very nich beach there. The only thing I didn't care for was I found the vendors to be very pushy. We're going back in April 2005 and plan to go to Uvero Beach. Others that went there had a great time. The other popular tour is to the ruins. We opted against it as we travel with our children and were told the bus ride was about 2 hours and it was very hot. From what I heard from those that did go, it was a fantastic sight to see.
Hope this helps.