Calakmul is a UNESCO World Heritage Site, and is considered to be one of the oldest and most powerful of the Mayan cities.
My suggestion:
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.
Summary:
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.