The ruins themselves were actually pretty cool. There was plenty to see there, and we even got to see a spider monkey up in the trees. The port of Costa Maya is incredibly run down and now that I've already seen the ruins, I can't think of any reason I'd need to get off the ship in this port. Disney should re-think stopping in CM.
We LOVED Cozumel. We hired a driver for the day and got to see much of the island. We drove through the jungle out to the side of the island that doesn't have electricity. It's amazingly beautiful out there. It must be the windward side, because there were plenty of waves.
We had our driver stop at a restaurant/beach resort where we swam for a few hours. I'd go back to Cozumel in a heartbeat. Also, my husband was able to get a cup of Starbucks, a welcome relief after semi-poor ship coffee.