The last time we were in the Grand Caymen, we went on a sting ray snorkel trip with a company that is so much better than the ones offered by the ships and had the best time ever. It was on a large cat and we did a second stop and the ride was worth the price in of itself. We have also done just a beach day when we were there previously and it was very relaxing and sooo inexpensive as you can do it on your own. In Jamaica we have done the falls at Dunns River and also a tour of the island and enjoyed both.
Enjoy your cruise !