One of the spherical capacitors is made to stand unstably. Electrostatic force causes the capacitor to wobble back and forth as it charges and discharges. A charged rod is held near one end.
Electrostatic induction causes the stick end to 'follow' the rod.
Styrene pellets in a flask undergo erratic motion when the flask is placed near the Van de Graaf. Rice crispies also work, flying around quite dramatically. Demonstrate the principle of electrostatic spray painting.