We went to Tulum on a ship's tour and it was very interesting. They will tender you to Playa del Carmen then bus you to Tulum. I understand Xaceret is nice too, but that may be an all day excursion, where as Tulum should only be about 3/4 of the day with plenty of time left to shop and relax once back in Cozumel. Don't miss Poncho's Back Yard for a cool refreshing drink. Strolling Mariarchi's and all.
Bon Voyage. p.s. If you bring a Video Camera, there is a fee at Tulum.