I have a friend that is an amateur magician and last year he decided he wanted to add a flea circus to his act. He bought a cd movie on how to make and use them and then decided I was the person to build it for him. It was an interesting project.
Here is a few pictures of what it looks like and what is inside the box that animates the stuff on the stage surface.
Stage level showing high wire and diving platform/tank – all the steps on the ladder move, the board bounces and the water will splash upward from the tank. The high wire will move back and forth making anything hanging from it look like it’s walking the wire. The 2 boxes on the side are the controls that stay behind the thing when the act is in progress. This all fits in another all style travel trunk.
http://www.packratworkshop.com/pics/fleac2.jpg
Under the stage view of the ladder and highware mechanisms. The ladder uses motors to twitch the plastic steps and cause the board to bounce. The high wire uses a cable connected to a drive screw and nut. The cable is connected to the nut and moves through pullys to the top of the stand poles. 
http://www.packratworkshop.com/pics/fleac3.jpg
View under the stage showing the battery packs, air cylinder and valve (causes the splash in the dive tank) and the center drive that will turn a pretend carnival ride and drive tine little cars around an oval race track. This thing was a pain to build.
http://www.packratworkshop.com/pics/fleac4.jpg
Close up view of the center drive and the magnets that follow the oval path.
http://www.packratworkshop.com/pics/fleac5.jpg
What the flea circus should approximately look like after painting. 
http://www.packratworkshop.com/pics/fleac1.jpg
These pictures do not show the tiny cars, cannon or simple prop. pieces that will also be used.