If you're going to Guatemala or Central America a trip to a Mayan ruin is almost obligatory. In Guatemala there are tons of sites with the majority of ruins still buried and eaten by mother nature. However, for the most spectacular of sites Tikal is the most recommended. This can be done over one day but can be extended to longer trips especially if you are into archaeology.
During a tour you're likely to see and hear howler monkeys, spot spider monkey and come across coatis (a raccoon like animal). The area is also great for birders as toucans can be seen. Of course the best time to see these animals is early morning before the sun, heat and humidity start. Tours start in the town of Flores and offer sunrise tours.
Given the hostility of the jungles that the Mayan civilization put themselves in most sites are still covered up by the trees and the vegetation that grows in the area. Excavations are an on-going process but the maintenance for the main sites is already overwhelming. While experts have different opinions on the numbers it's said that 80-90% of the Mayan structures has yet to be uncovered. If you are adventurous enough to go deeper in the jungle guides can easily organize this for you.