Another must see when exploring Chiapas is definitely the world famous archaeological site of Palenque.
Set in the middle of the tropical jungle, Palenque is one of the main examples of a Mayan city of the classic period.
This ancient city that saw the heights of its civilizations between AD 500 and 700, was discovered in the 18th Century.
I would strongly suggest to take a tour with one of the onsite certified guides, unless you have done much reading yourself, so you would be able to understand the site better.
It takes approximately 2 hours for a thorough visit. As a suggestion I would say to visit the site later in the day. It is less busy and you might get a great price for a guided tour.
The site closes at 5pm, allow enough time. We arrived around 3pm and the guide offered to take four of us at half the normal price.
As we did not have much time, Dom and I did a same day return trip to Palenque from San Cristobal which included a stop to the Cascadas de Agua Azul. We enjoyed it but it was a very long and tiring day.
If you have time available, do not rush your trip and spend couple of days in one of the hostels or guests houses close by Palenque. It is also a great chance to immerse yourself in the jungle and if you are very lucky you could spot (or at least hear) the local jaguars.