Parque arqueologico uaxactun, Flores

Uaxactún and its community are a must visit if you're already in Petén. It's just a 30 minute drive after the entrance to Tikal and 200% worth it. This archeological site is enormous, but more than that, it's a natural reserve, one of the most important green lungs in Central America. Everybody who lives here (less than 700 people) cares so much for the environment and their way of life is exemplary for the rest of mankind. Although there is no electricity, they rely on solar power for basic things. They are all very much in touch with their surroundings and extremely knowledgeable when it comes to the history of this site. I highly recommend getting a local tour guide, and go early in the morning to see howler monkeys in the canopy above you.