Western Caribbean Excursion Ideas

Here are the plans, advice and comments appreciated:

Grand Cayman - half day excursion with Captain Marvin Stingray Sandbar Tour

Costa Maya - nothing yet. Is there good snorkling near the port or should
we book an excursion. If so, any recommendations? What are the best things
to do there?

Cozumel - nothing booked but we thought we would taxi to Chanunkaaub
National Park for a beach day and some snorkling. Is the snorkling ok there
or would we do better to book a snorkling excursion?

Belize - Cave tubing with Reggie.

My wife and I have two kids, aged 17 and 18 that will be with us. I expect
my daughter and wife will likely want to spend some time shopping. Of the
above stops, which would be best for that? My son and I are pretty much
content to snorkle every day.

thanks

Curt