Toaster simple autonomous vehicular design (by autonomous I mean "goes forward all by itself" not "navigates its surroundings all by itself"). You will need a DC motor a 9 volt battery and a 9 volt battery connector. Your motor probably won't snap into the motor holder unless it happens to be the PARSIMONIOUS & PETITE MOTOR from American Science & Surplus, but the design should be easy to modify to fit whatever you've got lying around (Yes, I know it's a 6 volt motor hardwired to a 9 volt battery-- trust me it won't blow up).
Print out two chassis-lengths, one motor holder, two bumpers, three axels, four wheels and one each of the two gears. Snap bumpers and chassis-lengths together to form the frame. Push motor gear onto dc motor. Insert DC motor into motor holder with wires protruding out the back. Slide motor holder on one axel and motor-gear on another. Slide rear axel (the one with the gear on it) into the holes in the chassis-length farthest from the other two holes. Secure rear axel in place by snapping on wheels. Insert front axel and secure in place with remaining two wheels. Insert middle axel into square center holes in chassis. Glue center axel into place at both ends. Slide motor holder to center of axel and glue in place. Slide rear axel gear along rear axel until it lines up with motor gear and glue rear axel gear into place. You may also want to glue the wheels onto the axels, but if you do it will become permanently un-assembleable.