The ruins of Yaxchilán are definitely one of the most beautiful we've seen in Mexico so far. Set in remote Lancadon jungle right next to to border with Guatemala, you can only reach them by an hour boat trip from a small town here. It was almost like from some Indiana Jones movie - small jungle plaza with Mayan ruins, with howler monkeys making their loud noises, spider monkeys in the trees above us. And many many birds.
Definitely recommend going here if you're around Palenque :)