There really isn't anything unique at Grand Cayman other than the stingrays.
Lots of good snorkeling at Cozumel of course, and lots of beaches. A relaxed environment can be had at the Club Cozumel Caribe
. This is north of the town. Great food. It was nice and peaceful when we were there. I think it is still offering day passes. I know they were rebuilding the hotel after it was smashed by a hurricane. When you approach the place it looks like a war zone, but when you reach the beach/bar area it is spotlessly clean.