If you wander in Grand Cayman, make a right as you come off the tender pier and head down about 2-3 blocks to a place called Paradise Cove. They have snorkeling and swimming there with lots of fish and reefs plus beach chairs and lockers if you need them. There is also a great little bar/restaurant there with great food and cold red stripes. On your way there you will pass a Hard Rock cafe on the oher side of the street if you want to check that out.
For a party time in Cozumel, make sure you spend some time in carlos n charlies or senor frogs.....you guys may have fun renting a jeep for the day or maybe some scooters and tooling around the island. Stopping in beach clubs along the way.