jitterbugs
Jitterbugs are a variant of vibrobots (just like Bristlebots). They are simple to make from materials you have lying around (a great way to upcycle!), plus a battery, some wire, glue and a DC motor.
Check out the how-to from The Exploratorium. These are really simple to make, but the sky's the limit for how complex you can make them. Once you start the motor, your Jitterbug will vibrate all over the floor or table, moving pretty unpredictably. A fun variant to try: use a bucket and mount the motor underneath. Decorate like a monster. When you turn on the motor and set it on the floor, the resonance from the bucket sounds like a monster roaring, and it moves! You can also substitute uncapped markers for the legs, and create a Scribblebot. It will make interesting patterns on paper when you turn it on.
Check out the how-to from The Exploratorium. These are really simple to make, but the sky's the limit for how complex you can make them. Once you start the motor, your Jitterbug will vibrate all over the floor or table, moving pretty unpredictably. A fun variant to try: use a bucket and mount the motor underneath. Decorate like a monster. When you turn on the motor and set it on the floor, the resonance from the bucket sounds like a monster roaring, and it moves! You can also substitute uncapped markers for the legs, and create a Scribblebot. It will make interesting patterns on paper when you turn it on.