Here are some examples:
In the bee-eater section, here is a recreation of the rock face in which bee-eaters lay their eggs:
While you are looking at the elephant and rhino enclosures, your eye is caught by a realistic monitor lizard on the rocks:
Here's a display of a cobra with her eggs, something that I would definitely find it very hard to see in the wild:
The Zoo authorities have built these bird nests to encourage the birds to nest and breed (yes, they ARE built by humans not birds!)
And in the cheetah enclosure, here's a recreation of a kill hanging on the tree. I am pretty sure this is a recreation of the kill that Steve Bircher saw during his incredible 2000 Serengeti sighting of the female cheetah killing a Thomson's Gazelle, which I referred to this post :
The paths in the Zoo are filled with different tracks, of birds and mammals; indeed, to go the River's Edge where many mammals are, one has to follow the hippo tracks!
As I said...the St Louis Zoo is so well run....and all this for a fraction of the prices that other Zoos charge.