At Grand Cayman if you go on your own, there is Margaritaville Restaurant with a rooftop pool. We have done the Seven Mile Beach Break where we went snorkeling. We saw coral, fish, an eel, and a sea turtle. The water is clear here. Alot of people like to do the Stingray tours.
At Roatan people have suggested the Tabyana Beach Day. If you like to scuba dive, I have had family that went to Anthony's Key. The water is very clear here.
At Belize people have suggested the beaches because the water is clear for snorkeling. Xunantunich and the Altun Ha Mayan Tours look interesting.
At Cozumel people have enjoyed the Mayan Ruins of Tulum. The last time we went to Cozumel we went on our own and went shopping downtown and ate at the Hard Rock Cafe. If you go on your own, you need to ask the cab driver how much to downtown before you get into the cab. You should only have to pay between $10-$15. There is also a Senor Frogs downtown and at the pier by the ship.
There is a lot to do at each of these ports. It just depends what you would like to do. If you search each of these places on the internet, it may help in your decision. That is how I found out about Margaritaville and Senor Frogs. I also occasionally catch some of these places on the travel channel. Have fun whatever you decide to do.