When we visited Guatemala a couple of years ago, we spent time at Tikal, up north near the Belize border. The ruins there were wonderfully hidden, free from hoards of tourists, and large enough to give you a good sense of just how grand the Mayan civilizations used to be. The Coba ruins are the only set in the Yucatan that mimics the formations found at Tikal. It is believed that the families of Coba and Tikal had some kind of marriage bond, which explains the similarities in architecture.
The third day we were in Tulum, we drove out to the Coba ruins, stopping at Gran Cenote on our way back to the beach. It made for the perfect day trip - if you are heading to Tulum, I would highly recommend renting a car. It allows you to keep a flexible schedule and really explore the area.
Nohoch Mul, the largest pyramid formation at Coba is the tallest in the Yucatan peninsula at 138 feet. The best part about these ruins is the $3 bike rental option. Just do it. No where else can you zip around on a bike in a jungle, "parking" in front of different sections of the ruins and then hop back on again and bike to the next set.
After hiking up Nohoch Mul we bought an orange Fanta from one of the guys at the base of the temple. We sat there and chugged the ice cold soda, dripping sweat from the stair climb up (and bottom scoot down). Afterwards, our hands sticky with citrus syrup, we climbed back up on our bikes and headed to the next set of ruins.
What I always find interesting is how little of the actual ruins are uncovered (similar to Tikal), everywhere you look there are large mountains of rubble, trees growing out every which way, that just haven't been unearthed and reconstructed yet.
When we were finished biking the loop around the Coba ruins, we hopped back in our little rental car and drove back down the road to the Gran Cenote. It is part of the Sistema Sac Actun or White Cave System which is the largest surveyed underground river and cave system in the world. The Gran Cenote is fresh clear water with a soft sandy bottom. We brought our own snorkel gear and just swam around, checking out the cool stalactite and stalagmite formations throughout the caves.