Here’s a neat little project that anyone can do: make battery powered magnetic LED “throwies” that can be thrown and stuck to any ferromagnetic surface in your neighbourhood.
A throwie is made from a 10mm diffused LED, a small lithium battery, and a rare-earth magnet all taped together. They cost about $1 to make and the batteries last about a week.
Instructables has a page of step-by-step instructions to make your own throwies.
Also, check out throwies in action.