Project to school with arduino

I have to do a project for school that involves electronics. The project will then be presented at the end of each school juris. But I do not know what to do!
Accepted suggestions but the project can not be too complicated to do.
Tanks...

Make a magic trick: a led light with a hidden switch (e.g. tilt sensor) or sensor (LDR)

A door lock based upon a secret knock

or the ultimate project - Useless Box Kit from ThinkGeek - YouTube -

Do some sort of LED or POV project. They are relatively easy while being showy and impressive. Add a button so you have something to push. Use the buttons to change LED color or flashing speed or pattern. Start with something simple and add to it if you have time.

Make a game like Simon Says. Add different color LEDs and a piezo for sound.

What grade?
How much experience have you in electronics and programming?
What is your budget ($)?