Magnets are always cool, and I’m personally quite attached to my plasma ball.
How about asking them:
-Why do TVs need 3 colours, not 4 or 7?
-Why are the primary colours of light and paint different?
-Why doesn’t white+black = white?
And then showing them a spectrometer? Very simple, yet these are very common questions, even on the board.
Here are some pictures I took with mine: https://lh4.googleusercontent.com/-gmjOd9GzPp8/UYPUaZyn7eI/AAAAAAABQYE/HPp886rzdKo/s0/the%252520true%252520nature%252520of%252520light.jpg
I want to add in a plasma speaker.
Make ferrofluid (magnetic fluid) with laser toner and vegetable oil (though I’m somewhat doubtful this works as advertised; alternatively, there’s also places online you can buy the stuff).
A timeless classic is the flip top: a top that, once spun, flips itself around to continue spinning on its handle. Pauli and Bohr seem to have liked it!
We were certainly aware of the high temperatures required for ignition. In another experiment, we used the element required for thermite ignition to do something that resulted in the melting of a test tube. Nobody was ever injured in our school, as they took safety seriously, but we got to see some cool stuff.
As children we are told “do not try this at home” but as adults the advice is often “hold my beer and watch this!” 
For a less dangerous, but pretty science experiment, do chromatography. Use a not too dangerous solvent to show the different colours in a coloured ink stain. Use the chromatography principles to show different types of chlorophyl in leaves by grinding up different leaves in solvent and seeing how they migrate over paper.
Vis-a-Vis brand overhead projector markers are great for paper chromatography. Use white paper towels and water.
Probably a parity flipping top. IDK. Started on a new book that might actually explain parity. Susskind’s book seems like the scenic route that only gets you to the departure terminal. 
You can buy flip tops or “tippy tops” but a high school or college class ring will usually work. Start it heavy-side-down and give it a good spin and it will soon flip itself.
You’ll need a table, many bottles of Diet Coke and lots of Mentos
… and lots of help clearing up. :eek:
I actually had such an opportunity. I was working in the newsroom at a radio station when a school bunch came through. I had a network feed that came in, and the option of receiving it over a land link or a satellite feed. I turned them both on at the same time, to record them simultaneously, out of synch. I marked the tape at the spots where a tone started on each of the two feeds, measured the distance between the two marks, at 7.5 inches per second, and showed them how to determine from that and the known speed of light, the height of the satellite that relayed the feed.
Gyroscope
Cheap faster than light travel. I’d really light to be able to demonstrate that to a bunch of kids. Sigh.
Vortex rings. The Vortex Cannon - Student Science - YouTube
Oxygen takes up space: fill a shallow pan with water, put a lit candle in the middle of it (not so much water that the candle floats). Put a jar over the candle. The candle will run out of oxygen in just a few seconds and snuff itself, and then the water level inside the jar will rise as water comes in to take up the space of the missing oxygen.
And there’s the fun egg in a bottle experiment.
Common misconception, the water level rises because the air cools and shrinks once the flame goes out. e.g. Science Kits & Science Toys | Steve Spangler Science