Do the road with magnetic paint and add magnets to the cars. Playroom or little boy's room. I think the better idea would be to place this on the floor rather than teaching kids to drive cars on the walls.
DIY dog booster seat for the car. Cut out two bigger holes in the back for the seat belt to go through and tie a leash to the top. Sew a cube shaped pillow for the interior and a slip cover with elastic for the top (for ease of cleaning) and you're set!
Complete List of Things to Keep in Your Car for the Chronically Forgetful