We did this same cruise with daughters ages 14 and 12. No Mayan ruins in Grand Cayman. In Grand Cayman, we went on the Atlantis Submarine. You can do that very conveniently with Disney, or save some money by booking online directly with Atlantis submarines (do not confuse with Atlantis Resort in the Bahamas).
In Cozumel we went to Rio Secreto. There are a couple threads on here where people discuss Rio Secreto. It is an underground river tour, and it is fabulous. I don't know if your kids are old enough to go, but it was great. The logistics of getting there are tough enough that doing this one with Disney might be wise. Other people went to Mayan ruins from Cozumel and loved that as well.
We went to the Mayan ruins at Costa Maya. We enjoyed our tour very much. We once spent three days at a resort in the jungle of western Belize, touring Mayan ruins and sacred sites. Consequently, I think it speaks well of this Mayan tour that it held our interest and we learned a lot. There is not much else to do in Costa Maya, although another thread on this forum had someone mention they had a great beach day in Costa Maya.
Frankly, I think either Cozumel or Costa Maya would have a great Mayan tour. You can probably look at the other tour options and choose what you want to do, then do the Mayan tour at the other port.
