If you are interested in Mayan ruins and are in the Playa Del Carmen area, Coba is a must. It is an enormous city, still mostly overgrown, that gives you an idea what the ruins looked like when the explorers first rediscovered them. You can walk widened trails and rent bikes if you wish. Climb the pyramids and look at the views!  Plan on spending at least 2-3 hours because the pyramids are a distance apart from each other.