When you complete a scenario. Every guest in your park will turn towards the general direction of the camera and applaud. If you have a Balloon Stand, every guest who bought a balloon will simultaneously release them into the air.
It gets better if you make a point of making a ton of different Balloon Stands, each of them selling a different color.
With the ability to customize peeps it was only time before someone made a peep made to look like Chris Sawyer.