The colectivo, or public bus (read van), is easy to catch from the highway, and easy to hop going back to your hotel.  If you're in central Playa, go to Calle 2 between 15th and 20th Avenues to pick up the colectivo.  The cost of a trip from Playa Del Carmen to Grand Sirenis is 20 pesos and is a 20 minute ride with several stops on the way (if there is room for more passengers).  A one way trip to Tulum is about 25 pesos.  The locals as well as tourists use the colectivo.  The colectivo runs up and down Highway 307 all day.

There is also the ADO bus.  The main bus station is located at the corner of 5th Avenue and Benito Juarez (near the ferry terminal to Cozumel)  Buses to Cancun central, the Cancun airport, Tulum, Coba and other locations start from here.  The other bus station is located on 20th Avenue at Calle 14.  Buses from here generally go to other parts of Mexico.  This is a very affordable way to get from town to town, with fewer stops than the colectivo.   Fares are as low as 38 pesos ($3.80) to central Cancun.  It takes you almost anywhere in the Yucatan. This is recommended for those who have been to Playa del Carmen before.  Buses are large, modern and air conditioned,  with TVs (the movies they show are in Spanish) and very comfortable seats, much better than Canadian and U.S. buses.  Look for the ADO signs.

Alternatives to colectivos are either taxis or car rentals.  For the best rental rate at the airport, prearrange your car through the internet.  However, there are many car rental companies in Playa del Carmen if you want to rent your car for just a few days (Hertz, Avis, National, Budget as well as many small local companies which will generally be less expensive).  Executive Car rental is $20.00 USD per day when you book via the internet where discounts are available which can't be offered at the local office.  If you don't want to spend a lot of time looking around for a car rental agency, just ask your hotel for assistance.