The Fascination out of Jacksonville always visits Nassau and alternates between Key West and Half Moon Cay (pronounced "key") on the 5-day cruises.  If you can, take a cruise that visits Half Moon Cay. It is an awesome port with a beautiful beach and crystal clear water. It was much more entertaining for our children than Nassau was. There are floating objects in the water (e.g., a life-size alligator, shark, etc.) that are anchored to the ocean floor. It's fun to swim out to them and climb onto them. There is even a large trampoline inside a giant innertube that you can swim out to and climb on. It's exhilirating to jump on a trampoline in the "middle" of the ocean. My two children swam for hours and practically had to be dragged out of the water to go back to the ship!

Carnival provides lunch on the island that day so you don't have to go back to the ship for lunch.