The ruins will take most of your day, because the tour takes you oer to the mainland, as there are no real ruins on the island.
My favorite thing to do is the submarine. It is not far from the port, and most of the time, the ship will offer the tour.
Another fun place to go is Paraidise beach. They have a good place on site to eat, and will connect you with people who offer wave runners, and parasailing.
Cozumel is a fun place to go, and have many offers.
Let me know if I can help, as we have done all of the excurions offered by Carnival.