It is an incredible experience to hike through primary rainforest, along paths that are incredibly pristine and wild. You might come upon a beautiful waterfall or hidden caves.
You certainly stand a very good chance of seeing:
monkeys
sloths
anteaters
giant iguanas
exotic butterflies and other insects
Poison Dart frogs
exotic trees and plants
macaws
toucans and other birds
Note: lighting inside the forest is low, so it is a good idea to bring a long lens and fast film, as well as insect repellant.