We did a very simple experiment to illustrate how the planets orbit the sun. All you need is a foil pie plate and a blue marble.
We taped a circle to the center of the pie-plate to represent the sun. We pretended the blue marble was the Earth and then gently tipped the pie plate to start the marble rotating the perimeter.
Miss M figured out that you had to be very careful to make sure the marble stayed on path. She loved this simple experiment and has redone it several times herself since.