We took a tour through the cruise line to see Tulum. The ship docked in Play Del Carmen to let the tour passengers off, then went on to Cozumel. They tendered us back to the ship after Tulum. This took 1/2 a day, and we spent the rest of the day in Chakanaab Park. I highly recommend this. Great snorkeling, covered lounge chairs, very pretty beach. I cost $10 to get in. We took a taxi there. I forget how much the taxi cost, but it was minimal.
Have fun, whatever you decide to do.