Funny you should say this..
Our school science fair is this Tuesday.
My kindergartner is doing a 3d Magnetic thing, using a bottle of baby oil, steel wool, and loose magnets. Basically you put the steel wool in small shaving pieces in the oil. Then you use the magnets to see how the particles move around in the oil.
My son did his own physics type thing.
He made a maze out of plastic pieces, and got nine marbles. Before he started on the marbles he made a prediction to see which marble would go the fastest through the maze.
He used a timer, and timed them all.
Then he measure the marbles, and saw which marble was the fastest.. (it was the smallest one).