topics for physic presentations?

1) Make a solenoid big enough. Put it upward. Feed it AC power (not DC). Put a small round aluminium plate over the solenoid and it will levitate. You can try a cut from an aluminium thick foil pan from the local grocery.. This is the principle applied in electromagnetic (very fast) trains. This because aluminium is a non-magnetic conductor. The currents induced in the plate by the electromagnet make the same magnetic polarity and thus the plate is repulsed from the magnet.

2) Diffraction of light. Take a good (low divergence) laser pointer and aim it (fastened, not with your hand) at two razor blades (The old type) holded by magnets and make the blades as close as possible. You then put a screen (some white cardboard) in front of this assembly where you will observe the stripes of black and light called the diffraction figure. (The room should be dark). Diffraction proves the wave nature of light. Use a real laser from your school for better results.