1) Play with some of the rare-earth magnetic building toy sticks; their interactions as they approach each other should be visually interesting. I also have some spherical rare-earth magnets lying around somewhere, but they are insanely strong, and if I recall correctly, the force between two of these little bastards is inverse 4th power -- but what about one of them along with a ball-bearing? Is that going to approximate inverse-squared? If so, they might orbit each other!
2) Fun with a plastic slinky. Nick suggests stretching out the slinky, releasing one end, and then releasing the other just as the slinky approaches full compression. The dynamics might be very interesting.