UPDATE: I've found an appropriate "vanity" challenge in the first adventure, which I can cut out if needed in case the players take too long on the other parts of the adventure. For the third room, they will enter and will see nothing but hundreds of mirrors and the door behind them. Once the players look into the mirrors, a doppelgänger of themselves will climb out of it and begin attacking them. Enter combat! However, they will quickly realize that these doppelgängers are not really taking any damage from their blows, and when someone makes a low roll they will instead attack one of the mirrors. Smashing the mirrors is the only way to damage these beings. Only, the doppelgängers are only tied to a few mirrors out of the hundred (with each mirror wrecked that it is attached to bringing its HP down by 1/3), so they will need to break a lot of mirrors (which will prove to be quite fun).