If you staying in town, go to Allende Street for a ride to the Ruinas in a collective van for 10 pesos. If you are in El Panchan, flag down a van on the main road for 5 pesos. All people entering the area of the ruins have to pay a 10-peso mandatory fee for beautification of the area. Wear good walking shoes and bring water. The museum is free and the exhibits are worth the time for a history of the area. Only a small fraction of the site has been excavated, but there are many temples and buildings with hundreds of steps to climb to reach the top, which you must do for the panoramic view.
The most impressive feature of Palenque is the carvings in stone of Mayan gods. Retreat to the cool jungle surrounding the waterfalls and down the winding path. This is a wonderful Mayan site, with descriptions at the base of building explaining the art and Mayan inscriptions telling the story of the picture. Pack a lunch and plan for a full day with walking and steep stairs.