Free trees and other stuff
When you are in any zoo mode (campaign, challenge, and freeform) most zoos should have already existing trees(and occasionally things like flower beds, benches, etc.) at the start. All you have to do is left click the item you want and move it to where you want it.This saves money when you are on a tight budget and is really easy.
Run in Zoo Guest Mode and Photo Safari Mode: Since left-clicking the mouse makes you walk in Zoo Guest and Photo Safari Mode, and so does W or Up, you can walk twice as fast (aka run) by holding two of these.