The National Park of Palenque is a great place to visit in the south of Mexico.
The ruins are located a little off the city. Buses and Colectivos/Combis run all the day. Take one near Palenque's Bus station that says "ruinas" on its sign. Prices vary depending on spanish knowledge and driver's mood.
The archeological sight is huge. In fact it's bigger than the one in Chichén Itza and far less crowded. Go there in the morning to hear the howling monkeys and to enjoy the ruins in a calmer atmosphere. Don't forget to check out the "queen's bath" at the opposite of the entrance (a 10-15 minutes stroll).
If you wish to eat and drink during your stay, bring food and water with you. Prices are high at the entrance area.
There are several tour guides offering their services. Check out prices BEFORE the tour (if you want one at all).
Keep in mind that there might be events or gatherings at the time of the solstices. New Age followers should have their favourite turist season around those days.