This is a such a fun project. Steve Hobley make a “ball whacker” that uses a simple circuit consisting of a 555 timer chip and a photoresitor to create a feedback loop. When a ball (or a plastic egg) suspended from a string blocks the light source from the photoresistor built into the arm, the circuit triggers the arm to hit the ball, which exposes the light on the photoresistor. When the ball swings back, it gets another whack. The device feels alive.
An Intro to Cybernetics via the 555 Timer Ball Whacker