Unless you plan on just staying in Akumal, you will need a rental car to get around. The closest sights are at least a 20 minute drive. However, if you're not comfortable driving in a foreign country, there are plenty of taxis to take you wherever you need to go.
There are also colectivos, a local 15-passenger van service that runs between Playa del Carmen and Tulum. They're cheap 20 pesos per person, and usually full, but run very frequently. You can catch the colectivos off the main highway. If you are standing on the side of the highway the driver will flash his lights at you if he has room in his van. This is the preferred local way of getting around.
