AM peak detector is probably the easiest and primitive AM demodulator: it's basically made by a diode, a capacitor and a resistance. I implemented it when I was in the high school and I was making the first physics experiments.

The idea behind this demodulator is quite easy: the diode filters out all of the negative part of the signal, then the positive signal charge the capacitor and the energy is released in a quite constant way (R*C must be several order of magnitude higher than 1/f where f is the carrier frequency) during the negative signal "hole".