I went on the same cruise as you- Rhapsody of the Seas?
Key West- Just walk around, you don't really need an excursion. Try to get out at night, if you can. It's such a fun city!
Belize- My dad made us do some fishing thing. I heard the snorkeling was good. I heard that cave tubing was cool but really cold
Cozumel- Snorkeling is always fun. However, if you've already done the whole snorkeling thing, or want to relax and not sight-see, you might just want to do the all-inclusive beach thing. Cause if you think about it, even if you're on a beautiful warm weather cruise, you don't get to enjoy the ocean and sand like you would on a regular beach vacation.
i went on the glory and we went to the same places. key west just walk around and go shopping and to margaritaville. Then in Belize the cave tubing was unreal fun. in Cozumel we went to this beach that had like trampolines, blow up climbing things, sailing, snorkeling, A booze cruise but that would be bettter for your parents, and canoeing. Then of course stop at senor frogs or carlos and charlies. Hope this helps.