Potential and Kinetic Energy
Set up a basic ramp against a chair, on top of some books, or from a table to the floor. Hold the car at the top of the ramp, and release to demonstrate the two main kinds of energy. Potential energy is placed into the car when it's lifted from the floor, and that energy is released as the car rolls down the ramp. Ramps should be made of a solid, smooth and rigid material such as wood or plywood.
Friction
Using the ramp from earlier, you can make a more interesting experiment by placing some things at the bottom. Drop the car from the top of the ramp again, and measure how far it rolls this time. Now, place a sheet of cardstock at the bottom of the ramp, and let the car roll over that. Measure the distance it rolls. Place some sandpaper on top of the card stock, release the car, and measure that distance. Finally, try it with some carpet at the bottom. You've now learned something about friction: the card stock has a lot less friction than the carpet, so the car should be able to roll farther.
Inertia
Place a heavy stack of books a few feet from the end of the ramp, so that the car hits those books when it rolls from the top down. Demonstrate that once or twice, and then take a small ball of clay or putty. Place it on top of the car and pat it down slightly; don't squish it onto the car, but press it just enough so that it won't roll off. Roll the car down the ramp, and when it hits the books, the clay should fly off the front of the car. This happens because of Newton's first law of motion: anything moving is going to want to keep moving (until it crashes into a wall).
Angle
The optimal angle for the board is about 80⁰
