This is much easier to construct than a small bristlebot and has better performance (bigger pieces, no need for a special vibrating motor, more standard battery, longer autonomy, on-off switch added). It also allows for a rechargeable battery (highly recommended).
Step 1: Materials
9V motor (lower voltage motors could work too)
9V battery (rechargeable recommended)
9v battery clip
Piece of wire (any color. I got mine cutting in half the red wire from the battery clip in half, which was long enough)
Unipolar on/off switch
Step 3: Attach the wire to the switch
Step 4: Attach the battery clip to the switch
Step 5: Attach the motor
Step 7: Glue the motor and the battery
Step 9: Attach the crocodile clip
Attach the crocodile clip to the motor shaft adding glue on and between its teeth (this is to avoid the clip from being ejected due to the vibration of the shaft as it revolves). If the shaft is wide enough or has a plastic cover, the clip may attach firmly without the need for gluing). BE SURE TO KEEP YOUR FACE AWAY WHEN YOU TURN THE BRUSHBROBOT ON.