Heated Rock: for Reptiles and other Heat loving Animals
Big hollow Log (lying): a hollow Log that is lying on the Floor which is big enough for Aardvarks, Tortoises and other shy Animals in similar Size to hide inside of it
Hollow Log (standing): a standing Log or Tree with a Hole in it, that can be used by Animals like the Red Panda (maybe even make it more likely for them to climb in there to give Birth)
Bird House Food Enrichment for Primates and other climbing Animals: a Nesting Box that can be filled with Eggs, Fruit or Meat, for Animals like Apes and smaller Primates
Honey and Apples: Can be put on the Ground for Bears to eat
Climbing Tree with Food on Top: a Pole with a Platform on Top with Food. Can be used by Big Cats and Bears
(not my own Idea) Food Pump for Flamingo Lakes: a Machine that pumps the Flamingo Food directly into a Body of Water, which allows more natural looking Enclosures
Tire on a Chain/Rope: a Tire that is connected to a Chain or a Rope which hangs from a Tree. Can be used by Elephants
Leaf Bedding (temperate Biome): dry Leaves that Animals can sleep in
Plastic Barrel: a Barrel that can be used by Animals like the Polar Bear. Can be used on Land or as swimming Enrichment Item
Filled Cardboard Box: a Cardboard Box that is filled with Hay and Fruit or other Food. Can be used by Apes
Coconut: can be used by Apes and other large Primates which can try multiple Things to open it (for example throwing or banging it against a Rock). Sun Bears should also be able to use it
Source: https://youtu.be/6OJkjeVn764
Small Hammock: a Hammock like the one from the African Building Set (Main Game) which can be used by small Animals like Red Pandas, Pangolins, Capuchin Monkeys
Spices for Big Cats: Big Cats are sometimes given Spices like Curry for Example as Enrichment. They like to rub their Body against it
Harder to implement:
Jumping Tower for Saltwater Crocodiles: a Tower with a huge Piece of Meat hanging above the Water Surface. Saltwater Crocodiles can jump out of the Water to get the Meat
Blankets for Apes: can be thrown around or used to get some Privacy
Feeding Station for Deer and domesticated Animals: Guests can buy normal Food or Deer Crackers to feed Petting Zoo Animals or Deer
Enrichment that is already in the Game but should be made available for other Species too:
Natural Termite Mound: should be made usable for Aardvarks, Pangolins, Chimpanzees and Bonobos
Waterfall Enrichment: should be made usable for Crocodiles
Hammock: should be usable by other Bears and Primates
Edit: would be nice if the Saltwater Crocodile Jumping Tower could be combined with a Animal Talk