Small strobe light using a 5W luxeon star.
Runs for over 2 hrs from a 9V battery, fits into a altoids gum tin.
A fun project that cost under $50 and takes under 2 hours.
you also get to claim bonus geek points when your electronics come with a warning like:
"WARNING! Luxeon LEDs are extremely bright! Looking directly into a into a lighted LED will cause eye damage! Use caution when working with these LEDs"