Grand Caymen - book a tour on your own to Sting Ray City and Sandbar - many really good tour operators to choose from, we have used Captain Marvin and had a great time - book in advance - fill up fast.
Cozumel - depends upon what you like to do. If you want a snorkeling experience then book a ships tours to the Palencaar Reef - amazing. If you want to relax on the beach, do a little snorkeling, explore mini-ruins, maybe swim with the dolphins then grab a cab to Channunkaab National Park - if you want to do a dolphin swim you must reserve that in advance - books up fast. Playa Del Sol is a fairly new beach area- hearing really good things about this place. If you want to explore Mayan Ruins - book a ships tours to the ruins ( only because its a fairly long ride to get there and it involves a ferry)
Make some time to walk around downtown - maybe do a little shopping and have a killer Marquerita at Carlos'N' Carlies