Tulum is beautiful. It is the only Mayan ruin that overlooks the water.
Go back a few days in the trivia game and you can see a photo.
From Coz, you take the ferry across to Playa del Carmen -- about 45 minutes -- then Tulum is about 40K south. So, count on another hour to get on the bus, get there, etc.
Port days in Coz tend to be long so you will still have time to do something else.
Often a Tulum tour is coupled with a snorkel outing at Xel-Ha.
Don't know how much it costs.
If I had my choice, I'd club up with some other people, take the ferry, rent a car in Playa and go on my own.
pamda -- CM First-Time Cruises Editor