I had fun leading a workshop a few weeks ago on building a crystal radio. We used a simple design that incorporates a loop antenna (doubling as an inductor), a variable capacitor, a germanium diode, and an earpiece.
Two pieces of wood make a frame for the antenna. We nailed in picture hanger hooks to the ends to wind wire around to form an antenna. Then we used a few nails to hold the variable capacitor in place, and soldered all the components together.
The walls of the library we were working in dampened outside radio waves a lot, but once we stepped outside we could all hear some nice strong AM stations.