The arduino powers one LED when the button is pressed, the other when it is not:
My combination lock interface: A coin operated lock (the 'unlocked' green LED goes on when all three coins are in)
Tinfoil circuit under the hood:
And finally, my dream device:
It is an interface for 3d modeling/digital sculpting. It would plug into the computer, and project a hologram (on top of the tinfoil) that the user (wearing the stylish leather interactive gloves) would be able to sculpt and shape with his hands instead of using a mouse/tablet.