If you want to visit Tulum or Cancun from Playa, you can take a colectivo which is a small minibus. There is no schedule, they leave when they are full. It's 30 pesos to Tulum, 20 to Puerto Aventuras or Cenotes. They leave from Calle 2 between 15 & 20, across from the park.
If you are hungry before or after your trip, see my recommendation for a great cheap place to eat nearby,
The colectivos runs from Cancun to Playa del Carmen then from Playa del Carmen to Tulum, and back, all-day, every day. They are a van with air-con which holds around 14 people in at any one time. They have a coloured strip running down the side of the bus either red, blue or orange i see when i used them and also has the word colectivos on them. The running times on these is 5.00am till 10.00pm and esimtated waiting time is 5 minutes but there is so many your never waiting more than 2 or 3 minutes. From our hotel to playa del carmen it cost us $2.50 but in a taxi would cost $25, therefore there is a massive saving. We seemed to be the only tourist on this bus when we used them and talking to people around the hotel they didn't seem to know alot about them, it seemed to be for locals alot but we never got any issues from using these buses.
It would cost around $3 to run from Cancun to Playa del Carmem and the same from playa del carmen to Tullum. If you got off any time before them it might have worked out slightly cheaper like for us at $2.50.
I advise using these when in this area, they save you money and quite alot at that and the drivers seem helpful even though they speek little english.
Playa del Carmen was the easiest place I have ever been for transporation. This area has a great bus system and it was so easy. I took the ADO bus directly from the Cancun airport for about $8 USD and it takes you directly to 5th Avenue in the heart of Playa del Carmen. It was on time and clean. It took about 1 hour. In addition, instead of a taxi, try a colectivo...for about $2.00 you can get all around Playa and to Tulum and places in between. I fact, you can just stand on the main road in between Tulum and Playa and just wait for a van to pick you up. No reservations needed, just stand there, they will come eventually. It only took me a few minutes to ge picked up each time I tried it.
Don't worry about transportation on this trip...it will be easy.
You get the Colectivo in Playa del Carmen at Calle 2 Norte between Av. 10 and 15
At the Cancun Airport, look for the ADO employees, wearing a red shirt
In Playa del Carmen the ADO bus is 5th Avenue and Juarez. When you get off the bus you can probably walk to your hotel.
To get to Tulum from Playa DEFINITELY take a collectivio van. They pick up at the corner of Calle 2 and 20 Ave and leave every 15 minutes or so. Ask and they will drop you right at the road to the ruins (actually, they will drop you anywhere in between playa and tulum if you ask. Pesos only. Many of the locals use to get to work at resorts south of playa. It's about $2.60 US all the way to tulum
Colectivos are a great inexpensive way to get from Playa del Carmen down the coast to Tulum, Xelha, or Xcaret.
These are shared vans that leave when the van fills up, every 10-15 minutes. The cost to get to Tulum from PDC was 20 pesos, the amount of the fare was tied with how far you went. I think we were charged 15 pesos to get from Tulum to Xelha, I saw other people being charged 10 pesos to go shorter distances. You pay when exiting the colectivo.
Look for the white vans on Calle 2 between 15th and 20th Avenues.
Coming back from Tulum, Xelha, etc., you can stand on the highway and flag them down or look for them near the highway.
According to Lonely Planet, they run from 5 am to 10 pm.