Two coins are shown and can be examined if required. They can also be borrowed if preferred. One of them has a sticker bearing a spectator's initials placed on it. A handkerchief is placed over the signed coin and the plain one is positioned on top of the cloth which is folded back across it. A wave of a magic marker pen across the handkerchief, and immediately the cloth is unfolded to reveal that the signed coin is now resting on top of the handkerchief, and on the table below the cloth is the unsigned one.
Simple to do, this requires one common coin gimmick.