An agency two streets north of the cathedral, carrying a national tourism registry number (RNT 05070780774) — worth checking for in Chiapas, where a good deal is sold off a clipboard in the street.
Their long day is Yaxchilán and Bonampak: a drive to the Usumacinta, a boat upriver to a Maya city reachable no other way, and then the painted murals at Bonampak, which are the best-preserved Maya wall paintings anywhere. It is twelve hours and it is worth it.
They also run the Sumidero canyon, Agua Azul and Misol-Ha, Palenque, the Lagunas de Montebello, and the highland villages of San Juan Chamula and Zinacantán.