Besides the wonderful beaches, friendly people, interesting mayan pyramides, you should not forget to watch the wonderful nature and be aware that nature is somewhat damaged because of tourism. There is a place on the highway from Playa del Carmen to the Airport which is called Crococun.
That place is great to visit, it will take you half a day with good english speaking guides and the entrance is no more than 15 usd. You will meet great part of the local animals like crocodiles, spidermonkeys, deers, mexican naked dogs, tigrillo, ocelote, birds, snakes, turtles and more. It is an interactive tour, wonderful!! Besides that is it great to know that what you pay, will be at the benefit of the local animals. People are very nice there, most of them work for a low salary or even as a volunteer because of their love for animals and nature.
Do not miss this!! and have fun.
Josey