We've done this trip several times. The warm, relatively calm waters of the W. Carib. are a lot of fun for all. Here are my comments:

Cozumel - Chankanaab is a great government-operated park. http://www.cozguide.net/chankan.html Two other beach clubs popular with cruisers are Paradise Beach http://paradise-beach-cozumel.net/ and Mr. Sancho's http://www.mrsanchos.com/ It is easy (and cheap) to get a cab to/from any of these places. We, too, have rented a vehicle and done the island circle. It's a lot of fun, and the beaches on the other side of the island are fairly "undiscovered" by the masses of cruise tourists and other day trippers.

Grand Cayman - Seeing the stingrays is really a must! Out in the middle of the ocean, there is a sand bar with water that is waist high. You will stand up, and hundreds of stingrays swirl around your legs. It is a most unusual and delightful experience! You can make a trip out just to see the stingrays or combine it with snorkeling stops, which I highly recommend as some of the best snorkeling we've ever done. The independent operator we used was Capt. Marvin http://www.captainmarvins.com/

I don't have a rec on the Jamaica stop, except to say most people rave about the Dunn's River Falls experience.

Note, the discussion forum at http://www.cruisecritic.com is, hands down, the best place to learn up-to-the-minute cruise information. The Ports of Call section features each of these W. Carib. stops, and then there are forums dedicated to the cruise lines that are great sources of ship-specific info.

Diana Ball near Houston, TX