Write down what you see, most creative "desire" wins.
mirror decorated with Erised inscription
paper
pens/pencils
Decorate an existing mirror with the inscription from the Mirror of Erised.
You can buy an inexpensive mirror with wide frame that is then spray painted gold, then carefully stencil the inscription around the edge.
Have the participants look in mirror then write down what they see on piece of parchment or scroll-look paper.
Later, a prize is awarded for the most creative description or "desires"
Other variations especially with those in character costume might be to write down your desire as the character would (ex: "Trevor" wrote that he hopes he gets his memory back).