I would pick something very "physical" like robots, music, graphics, 3D
printing...
Maybe combine the above with a Rasberry Pi or other exposed computer, so
that the educational demo is not judged by the slick standards of some
rather impressive laptops/tablets/smartphones/gameconsoles that they
already see at home.
Or, take something they already know a bit, like a tablet, and show them
that they can create their own things for it (rather than being passive
consumers of something someone else has made).
If you want to do robots, and have a Roomba and a cable, you can always
use "http://www.neilvandyke.org/racket-roomba/";, though there is also
non-Racket robot platform software that includes simulators with 3D
graphics.
Or show them "http://scratch.mit.edu/"; or "http://www.squeak.org/";.
Neil V.
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