Take a small cleaning brush (different types may work).

Test if it slides well over the floor, if it keeps balance, you have a good brush.

If it tips over, you can cut the brush hairs into a more stable shape.

Solder a small DC motor to an AA battery holder, add a switch in between. Test the circuit on your desk.

If it works, add some assymetric weight to the motor axis, such as these two bolts :

Test the vibration of the motor with the asymetric assembly. If it moves slowly on your table, you did well.

Now glue the motor to the brush, leave some space for the switch and the battery holder.

Glue the switch and the battery holder to the brush.

Testdrive your robot, check the balance and reposition the battery holder until the robots follows an 'intelligent' trajectory

Insert a rechargeable battery, and save the environment !

Your project is ready !