I have 2 girls, age 8 and 14. We did this cruise in 2010 and their favorite port adventure by far was stingray city. They are still talking about. As a matter of fact, we were discussing excursions for our June 2012 Eastern cruise and how disappointed with the choices we are. My 14 year old, was "I wish they had something cool like Stingray City - that was the most fun ever!".
We did the stingray city, turtle farm and hell tour through Disney. They really liked the turtle farm too. Hell could be totally skipped, but alas it is part of the 'package'.
We did not visit Costa Maya in 2010, so can't help there.
My youngest was quarantined in Cozumel, so all I did there was shop with my oldest near the dock. It was HOT!