The ruins at Coba are thought to be mostly consistent with the 600 to 900 AD. or Classic period. Although there is evidence of ruins which were built before and after these dates, the fact that so little of this 80 square mile site has been excavated makes a complete history premature.
As the ruins at Tulum are the most popular in the region, many tourists choose to bypass the ruins at Coba. This is a mistake. If you have the time to go, it is well worth the short side trip (26 miles from Tulum). It is true that most of this site is still buried by a thousand years of jungle growth, but Coba is thought to have close to 6,500 structures and a population which once topped half a million people. . . There is so much more to do, but what there is to see here makes for a nice day trip.
The sections of the site which have been excavated are spread over several kilometers, so be prepared to hike (or hire a bike to drive you). In this region, the heat could make such a hike draining . . . but the roads to the various sections of the site are in the midst of a thick jungle canopy which provides shade and makes the hike more like a pleasant walk in the park (bring water anyway). If you come in the rainy season, be sure to bring mosquito repellant. Welcome to the jungle.