Maya Biosphere Reserve (Reserva de la Biosfera Maya)
The Petén lowlands in which the Maya Biosphere Reserve sits are classified as either Tropical or Subtropical Moist Broadleaf Forest, which means it will be hot and humid—much more so than Antigua and Lake Atitlán. It is coolest (relatively speaking) from November through February, when you may need a light jacket. Rainy season is steamy, and runs from late May through October; bring mosquito repellant.
Remember that while the reserve is protected on paper, the Guatemalan government has other pressing problems (poverty, narcotrafficking, corruption, etc). They can’t stop all the squatters who hunt, log, and mine the wilderness, or raid unprotected ruins for Mayan artifacts. Use your best judgment as you travel responsibly through the region.