Cozumel has become very commercial these days, you are bombarded with shlockey touristy CRAP, and high priced everything. If you have been there once.. No need to go back again. Key west is a MUCH better stop.
Belize is an interesting port, lots of good excursions, Some reasonable shopping can still be done there. but if you have No excursion to take. you wont like the city, it is very 3rd world. Dirty, and run down.
We took the river wallace altun Ha tour.. that was nice.. lots of wild life and history..