There are plenty of incredible ruins to explore in this area. Chichen Itza is a popular one (which always makes me think of the "Chichen Itza Pizza episode of Backyardigans), but it is a good 2 1/2- 3 hour trip each way and I wasn't quite sold on the hype. This article was helpful in determining whether it was worth the trip or not for us and ultimately we decided that there are other ruins closer by that are well worth a stop.  We combined out Tulum tour with a tour of Coba and we absolutely loved it! I would highly recommend renting a bike (or hiring a rickshaw) that are available when you get there, it was such a unique experience and allowed us to cover more of the massive expanse of ruins in a shorter amount of time. At it's peak it is estimated that it had a population of 50,000 people, this area hasn't been as excavated as other sites in the region and many pyramids are still covered in trees and vines so it just gives it an added element of authenticity. Coba has the largest network of stone causeways in the ancient Mayan world, there are 16 of these stone pathways open to the public and over 50 that have been discovered. It is home to Nohoch Mul, the tallest pyramid in the Yucatan and one of the few pyramids in the area that visitors are still permitted to climb. After an intense work out to get there, you are rewarded with a serene view over the jungle canopy. There are major safety concerns since the steps are very narrow and steep so wear appropriate footwear and use the rope to hold onto as you climb. Other points of interest are the "La Iglesia," the church, the many large stone carved monuments called "stelae" and the ball courts that were used to play an ancient Mayan ball game.