Calakmul is a UNESCO World Heritage Site, and is considered to be one of the oldest and most powerful of the Mayan cities.
To fully enjoy the site I recommend that you rent a car, sleep the night before in one of the hotels in the area, and leave very early in the morning. You need a good half day for the visit.
In the south of the Yucatan peninsula, on the road from Campeche to Chetumal, Calakmul is completely isolated.
Having visited really touristy pyramids like those at Chichén Itzá and Uxmal, I was able to appreciate how magical such a historic site could be.