Hi, we just got back a week and a bit ago and we did 2 fantastic excursions with our 9 yr old daughter, the first is xcaret , a huge eco park where you can swim with dolphins, snorkal in the cenotes, see lots of wild animals, see horse shows, blimey, I can't remember, there is loads to see and do, you can stay for the night show, which is amazing, tells the story of mexican history from the mayan gods to modern day mexico.
The other is xel-ha, a natural laggoon, this is fully inclusive, so all food and drinks are included, you can snorkal, bike ride, swim with dolphins and manatees, float down a lazy river, jump off a cliff !!( its not very high)
If you google these, they both have websites which tells you all about them.
You really will have a great time, mexico is amazing