Hi - RCI has a great excursion in Cozumel - Save a Turtle. We did this when my daughter was 6 and she loved it. We booked this through RCI because it was not offered anyway else at the time.
http://www.royalcaribbean.com/shore...horexByPort&ProductCode=CZT1&DestinationCode=
Another time in Cozumel we did the dolphin push/pull with RCI. We did this through RCI because it was just my daughter and me traveling and it was convenient. (You can do it on your own.) It was a lot of fun - we enjoyed the beach, spent some time with the manatees, and saw the sea lion show.
http://www.royalcaribbean.com/shore...w.do?sourcePage=shorexByPort&ProductCode=CZ79
And yet another time in Cozumel we took a taxi and spent the day at Paradise Beach. It was a great day. The pool is gorgeous - the biggest in Cozumel. The food at the poolside restaurant was excellent. And of course, the beach is beautiful.
We've been to the Bahamas a lot. We've done Frommer's walking tour, gotten the Hilton Day Pass a few times (nice pool, good food and great view of the ships from the beach) and did the dolphin encounter on Blue Lagoon. We loved the boat ride over and back. We didn't have much time on the island other than the dolphin encounter. The Cozumel dolphin experience was better but you can do the beach at Blue Lagoon if you want. I think that would be a nice day.
In Costa Maya we did the Chacchoben Mayan Ruins tour with Native Choice - excellent company. We booked this directly with Native Choice. My daughter was only around 4-5. There were 2 other couples each with a child in our van - all under 8. Native Choice knows how to engage the kids. Once we got to the ruins there was plenty of open space to enjoy, steps to climb, and monkeys in the trees.
It all depends what you'd like to do. Let me know if you have any questions or want additional information. We love the Oasis class ships!