OK - here you go. At Cozumel, do the dolphin swim thing. My wife did and had a fabulous time. While she did that, I wandered about Cozumel. I made the unfortunate mistake of buying about 4 XL T-shirts. After they were washed, they were smaller enough to fit a Barbie doll.
In Grand Cayman - we loved this place - go snorkle with the Sting Rays at Sting Ray City. My son and I did this and loved it. Gorgeous area - nice boat ride.
Note, take heavy duty sun block and put it on. Don't spoil your trip because of sun burns. Wear a hat and take sun glasses out to see the Sting Rays. The water will probably be low enough that you can walk around - wear a hat and sun glasses for sure. Also - right off the dock I found the neatest antique shop. Go in there and buy yourself an old compass. It's a helluva lot nicer gift that will always cherish and reminder of your trip there - compared to the cheap t-shirts and other junk sold on there.
Jamaica - go on the Bremer's Plantation tour and then go to Dun River.
Now the sour note: Jamaica is littered with garbage and Cozumel is not a whole lot better. I'm not saying they have to be pristine clean but for the love of Pete, somebody needs to clean up the litter.