I went to Playa del Carmen briefly when I was in Mexico last year. We were in an AI resort, but went out every second day to see something new. So for historical stuff, you can visit Chichen Itza, Coba, and Tulum - they're all within a few hours. I can recommend Akumal for swimming with turtles and checking out the coral reef - time it to visit first thing in the morning, or later in the afternoon (it's about 40 minutes south, on a collectivo - the local minibus system which is super cheap and easy to use). Turn right instead of left at the junction for Akumal and you'll find a good little diner run by a Canadian guy - they have a wings and ribs night. Also, if you want a really cool Cenote trip, try Cenote Santa Cruz (the pickup is in Akumal as well) - they aren't the greatest at answering emails, but the two lads who run it are a good laugh and the cenote itself is amazing - even my wife, who doesn't like caves found it fascinating.