This was one of my first Arduino projects. I disassembled a remote control toy car and replaced its guts with my own Arduino microprocessor and motor controller. I was able to reuse the servo, motors, and battery compartment from the existing car. The car was controlled via bluetooth from my Android phone.
View of the car with the cover off. Four main components are stacked on top of each other. From the bottom up: Arduino microprocessor, motor control shield, bluetooth shield, bluetooth module.