When my daughter was 8 or 9 we did the zipline in Belize, loved it!
Tulum is a great trip in Coz - lots of walking. Also Chankanaab National Park is great, dolphin encounter is worth the money too but the whole park is nice. You can do this with the cruise line or on you own.
Costa Maya we did the pool there at the port, kids had fun and we just had drinks and realxed.