I am not sure how much time you have. Take ferry to Play del carmen. In centre of town you can get on new air cond mini van that mostly locals use . They are white with Green letters. they say Playa del carmen- Tulum on the side. For the equivalent of about $3 USD one way per person you can take this to Tulum. They come every 15 minutes or so and the distance is between 50-65 Kilometres, so allow an hour(Warning wear seat belts all Mexicans drive fast. That means Taxis, tour buses, locals everyone)
Last year I paid $3 USD to get in to the ruins. Basically you can do this trip for about $10-15. each plus cost of Ferry to Playa.
I picked up the mini van on the Highway when we were staying in Puerto Aventuras and from there it is 35 kilometre to Tulum. The hotel taxi wanted $75 USD per person, cost me $7 for the whole trip. I also took the mini van from the highway adjacent to Puerto Aventuras to Playa Del Carmen on another day .
Nice thing about doing this way is you can go, see the ruins, and leave when you want to . They are nice but not awesome and certainly not worth the money the Ship charges.
I would say 1 hour to get over on the Ferry, 1 hour to 1 hour 15 minutes each way to Tulum and 2 hours at the ruins, so about 6- 7 hours on your own. If your ship does not leave until the evening you will be fine.
DOn't miss the Mexican guys in costume in the town of Tulum, they do some sort of dance and acrobatic feat around a very tall post, and lower themselves upside down. hard to explain, must be seen.
There is a beach at playa that you can use and swim in while you wait for ferry
Good Luck and have fun
