Edzná is the most accessible Maya site from Campeche — about 60 kilometres southeast on a good road. It is a medium-sized ceremonial centre with an impressive main pyramid and far fewer visitors than Chichén Itzá or Uxmal. Half a day is enough.
Calakmul, in the biosphere reserve near the Guatemalan border, is a full day's expedition — 350 kilometres round trip on long jungle roads. The site itself is enormous, the biodiversity of the surrounding forest is extraordinary (jaguars, spider monkeys, toucans), and the absence of crowds makes the experience unlike any other Maya site in Mexico. Start before dawn, bring food and water, and plan to spend several hours at the ruins.