We're going on the Sea Princess in January too and I've been trying to choose things to do. Have you checked out the Princess.com site on Captain's Circle for the shore excursions?
I wanted to book the catamaran that I've read about to Dunn's River Falls but doesn't seem to be listed. May do Stingray City in Grand Cayman but tempted to follow other advice and book that by ourselves from recommended sites. Cozumel may be a spot we either just wander about the town or take a cab to the nearby ruins or snorkelling. I will have my seven year old with us and I think these may be good for him too.