Cozumel - spend the day at the beach - Chanunnkaab National Park has a nice beach and the dolphin swims are here. Playa del Sol and Mr. Pancho's are also great beaches with lots of activites. If you are into Mayan Ruins then book a ships tour to the ruins. Shopping tours and native dance show tours are a waste of your time and money.
Grand Caymen island - definately book a tour to Sting Ray City/Sandbar, no snorkeling involved, you stand in the water and the rays swim all around you, you may pet and feed them if you like - amazing. You could also just hang out at 7-mile beach, or take a ride out to the turtle farm.
A rafting trip down a rain forest river is very relaxing and interesting. Different lines seem to have different names for it like Mountain Valley Rafting or Rafting on the Martha Brae. It was our most enjoyable excursion.