We didn't book anything on the ship for Grand Cayman. As soon as we got off the ship, there were tour vans waiting to offer tours. We took one that was $20 per person. They took us to the Turtle Farm, Hell, and the Tortuga Rum Cake Factory. They also pointed out many sites along the way. It was fun, but not real exciting. (You need to scroll down to the destinations part of the message boards. You'll get a lot of information there about what to do in each of the ports.) We then shopped a while, but let me warn you, Grand Cayman shopping can be expensive.
I usually shop in Cozumel, but there are many other great things to do there. Paradise Beach is supposed to be very nice and I plan to go there in December when we take our two daughters with us. Chankannab (spelling?) Park is great, but I'm not sure if they are open, yet. They received tremendous damage from the hurricane.
We were in Cayman last year, and wanted to go snorkeling. Some friends of ours referred us to Captain Marvins, a local snorkel company, and we couldnt have been happier. They took us ( a group of 8) snorkeling to a reef where no other companies were, they also took us to Stingray City, once you get there, you can see how crowded most of the boats are, with Captain Marvins, our boat had 14 people, it was FANTASTIC!!! Here is there website, DONT BOOK THROUGH YOUR CRUISELINE (if that is what you are doing), we all saved a ton of $$ by booking through the company. As an added bonus, they videotaped our diving, dubbed in some local music, and sold it to us. After dividing the cost by 4 couples, and copying the DVD, we had a GREAT & inexpenive reminder of our trip. I definitely reccomend... http://www.captainmarvins.com/
Hard to beat Stingray City in GC, but don't leave w/o buyin some rum cake. Too much to do in Cozumel for one day. We like Chankanaab Park (no excursion required) but Ruins at Tulum was unforgettable and that excursion takes you into Playa Del Carmen and you could spend the entire day THERE. Too much to do in Cozumel for one day, better plan to go back.