This is a very simple robot that uses a simple switch as a sensor and stands on only two wheels with inverted pendulum mechanism.
When the robot is going to fall the motor starts and moves the robot to the direction it is going to fall, so the motor torque about the center of gravity that is higher than the motor makes the robot balanced.
To make this robot you need following parts and tools:
small electric motor
(or a motor with gearbox)
some sheets of plastic to make bearings and the robot neck
two battery holders
4 AA batteries
one button cell
one SPDT (single pole double throw) switch with a metal lever
one toggle switch for the on/off switchs