You will have a great time!!! Your ports are great! Don't listen to what other tprople say about this or that. Just make the experience yours and you will have a great time.
Here is what I would suggest.
Cayman - Sting Ray City
Belize - Go Cave Tubing or to the Beach
Cozumel - Do some shopping then Hit the Beach
Costa Maya - Don't know, haven't been there
BBe sure to check out www.PortReviews.com
for lots of info on these ports. Don't worry. How hard is it to eat all you want, enjoy entertainment, dance, laugh and have a great time?