I did mine on name-learning tricks. Another guy at my audition talked about gamma rays. There was a girl who talked about taking care of your pet gerbil (and she rambled on, and on, and on, and on . . .).
Honestly, it doesn't matter a lick what you teach as long as you are engaging. You can teach them how to make the perfect PB&J sandwich, or how you dip an Oreo, or how to study in bed without hurting your back, and they would be fine with it.
As far as the boardwork, I don't think they look at that too much. Just giving a basic outline should be enough, like this: 1. Remove Oreo from bag. 2. Unscrew Oreo. 3. Dip. Etc. Or you could draw a picture of an Oreo being dunked and your smiling face. Just make sure your handwriting is legible, and don't talk too much to the board.
Basically, I think they're looking at your potential as a teacher more than your message. Make eye contact, smile, speak to different people in the room and move around (but don't pace), and just teach it. Don't go on too long either, as the gerbil girl did. At the Kaplan center where I worked, most people made it past the initial auditions and into training. The ones who got classes were those who either had seniority (i.e., worked there the longest) or who proved themselves to be solid teachers in the actual training sessions.
Good luck!
