In Jamaica, you should make sure to go to Dunn's River Falls - it is amazing. You and your group will climb the falls with your guide - it's well worth it. It's a bit physical but I'm very out of shape and I did it (was sore for a day or two afterward though). Take a water camera for amazing pictures! I didn't do this on a cruise, so I"m not sure which tour companies offer this tour, but I'm sure others will come up with tour names (I think there's a company called Pete something that I've heard good things about). In Cozumel, it is known for shopping and I think snorkelling. We did a bit of both, since you're usually there for a full day. I've also heard Chankanab (sp?) park in Cozumel is nice. In Grand Cayman, you should plan to do stingray city, and if you're looking for a good tour company, my recommendation is Nativeways ($25 per person). Hope this helps.
DLK - you have picked some great ports. Jamacia - Dunns River Falls is a great fun time for all ages i recommend booking a tour wit Peat Taylor that includes some local stops and a trip to the falls. If you want something totally different consider booking a rafting tour of the Martha Brae River - beautiful, lush, tropical scenery, get a taste of island life.
Cozumel - if you like to snorkel this is the place, consider booking a ships tour to the Palencaar Reef (amazing) . For a great beach day consider Mr. Panchos or Channunkaab National Park (Dolphin Swims are here - need to book in advance) If you are into history and the Mayan Culture consider a trip to the ruins. Make some time for a stop at Carlos & Charlies for a marquerita! You can walk around downtown and do some shopping - gems & watches are good deals.
Grand Caymen - Sting Rays are a must do! many tour companies to choose from. Other options - spend the day on 7-mile beach , tour the turtle farm , scuba or snorkel some of the reefs and shipwrecks.
WAIT!!!! Don't go with a group to Dunn River unless you are not very physically capable of walking up a meandering waterfall in a natural stair like fashion. We took a taxi from the dock, got there quicker...passed other "guided tours" stopped to take great photos when/where we wanted and chilled out in a nice little pool of water for a bit before hooking up with our cab in the parking lot as we had pre-negotiated. He then took us to the town market...picking us up 1/2 hour later to head to a local beach where we negotiated with a young man to take us out to the reef for a snorkel. I think we paid $25 for five of us.