1. Try putting a handful of paper hole punches on the top of the generator and watch them fly off
Why? The paper absorbs the electrons that are on the Van de Graff generator and each paper hole charges up. Once the paper is charged up it repels away from all the paper around it that also has a negative charge and away from the dome of the Van de Graff generator because it also has a charge. So the paper flies off in all different directions.
2. Hold the end of a long flourescent tube and bring it down above the VDG to touch it. You'll notice it will pull the light towards itself, then shock it and it will light up and it will push the light away.
Why? At first the light is neutral and then as it's held above the VDG it polarizes, giving the light a positive charge and the VDG has a negative charge. We know that opposites attract, and that is exactly why it is pulled towards the VDG.
It lights up when it sparks because the electrons travel through the tube and through you to the ground. As the electrons travel through the gas in the tube they excite the atoms, and just as soon as the atoms get excited they loose their excitement. When the atoms make this change they release energy in the form of photons, or light.
The VDG pushes the light away because now the light is more negatively charged, and the VDG get's negatively charged too. These dissimilar charges push away from eachother, since you're holding the light you can feel this brief push until the light repolarizes. (The reason also might just be a perception issue, if you just feel a pull and then the pull goes away, that may be felt as a push.
3. Hold your hand several inches from the VDG and put a small piece of paper between your hand and the VDG, you will see the paper "dance" back and forth between your hand and the VDG.
Why? When the paper contacts the VDG it gains a charge so it is pushed away. When the paper meets your hand it discharges to your hand and then becomes positively charged again and is strongly attracted to the VDG so it jumps back down to the VDG and recharges and the process starts over.
Put the Van de Graff on a table, then stand on a plastic 5 gallon bucket and hold one hand on top of the Van de Graff generator. This enables you to do a variety of tricks.
1. Your hair will start to stand up.
2. Point your finger after you've gained a good charge and you will see a light cone of little electric sparks shooting off your finger.
3. Hold some water in your hand and allow it to drip off your finger. You will see that the drip at the end of your finger comes to a tip instead of a bulb or raindrop shape. You'll also notice that the drop doesn't fall straight down, it falls down and is pushed away from you in a curved path.
4. Hold a flourescent tube and watch it light up.