There are a lot of fun experiments you can do with neodymium magnets. Take the VWR Gaussian Gun for instance: it comes with magnets on a linear rail and steel balls. When fully aligned and properly arranged, the steel balls accelerate 3/4 times their original speed with the pull of magnets.
This kit comes with everything you need to teach magnetism, motion, and Newton’s Third Law of motion. It ships with 10 x 3/4″ steel balls.