Our good friends over at Lifehacker found this video tutorial gem on how to create your own metal detector on the cheap.
Essentially, by just sticking a calculator on the back of an AM radio you can create an annoying tone sound when you put the two near a metal object. There's something about radio frequencies and digital signals or something, but who cares how it works when you know that it does work.
As far as hacks go, you won't get much easier than this. Plus, you've probably got the necessary components somewhere in your garage. What better way to spend your evenings?