This homemade metal detector is one of the easiest projects I've seen. All you need is an AM radio, tape, and a calculator. You don't have to take anything apart or follow any complicated directions, all you do is tune the radio, stick them together and let the reaction happen. As your device moves close to the metal object the radio waves from the calculator reflect off the metal and are heard as a loud beeping on the radio. The beep is intensified as you move closer to the metal object just as it would on a commercial metal detector.
Hack together one of these next time you're searching for a hidden treasure, or just give it a try for fun. The video results are impressive and it uses materials you have, takes little time, and is super simple.