Not really a craft idea, I would keep one for drawing and writing messages on peoples bathroom/dresser mirrors when you stay the night but leave before they wake up or they are gone before you wake up.
I'm not insinuating that you have fleeting encounters - it works as well with friends. ^__^
Else use it for body painting - as long as it's not on your lips there wouldn't be any harm in putting it on.
It might also work for coloring wax for candle making.