I've been to both Cozumel and Grand Cayman several times. In Cozumel, the Tulum tour is worth doing if a bit long, as it's an absolutely beautiful place. My daughters liked the Dolphin Swim at Chankanaab park, and my son liked the Atlantis submarine. If you dive, well, all the ports probably have great diving.
The one must-do tour (in my opinion, of course) in Grand Cayman is Stingray City.