That would be interesting...do it on cracking WEP wireless network encryption. It would be pretty easy, all the documentation you'd need is online and can be found with a simple Google search.
Just plug a wireless router in, set up a WEP network with a weak password, and have another laptop be transmitting to it while you crack it on your own laptop.
The tool you want is called aircrack-ng. As far as I know it only runs on Linux. You could definitely set something like this up:
That can crack a password very quickly (a couple minutes max) so it should be a good demo.