I just got back from a week-long vacation in Cancun. I had a great time in Playa del Carmen, Isla Mujeres, Coba, Puerto Morelos and Cancun.
I highly recommend Coba. It’s about 30 miles inland from Tulum and offers rustic views of Mayan pyramids and ruins. Coba thrived in 500-900 A.D. and was linked to Chichen Itza by a common road. The city had more than 50,000 inhabitants. Not many turists here and many ruins are overrun by jungle. 
The ruins are spread out by 1/2 a mile or more. We got around on bicycles. Also, be sure to climb Nohoch Mul. It’s the tallest pyramid in the Yucatan (42 meters). The view on top is worth it! That’s me to the left after the long climb.