Margaritaville is a bar, w/souvenirs. You want to be sure to do Dunn's River Falls. It is really a sun thing to do. Be sure to take water shoes, or old tennis shoes to wear. They do have them to rent, but I'd rather wear my old shoes, then throw them away after.
Grand Cayman--I second stingray city. You can get it cheaper to book on your own, we just got it when we went to port in Grand Caymen. You can book on line with several private companies, check the boards for the names.
Have a Great Cruise!