I had this idea at a halloween party, but there might also be other, more serious applications.
What if you could poke out your tongue at someone and display a certain design/words, and the next time your tongue comes out of your mouth, there's something completely different written on it? In theory, there are many (more or less convenient) options to achieve this, like putting a (mini)printer into your mouth, or hiding a tiny projector in your nose to project the image on your tongue once you stick it out.
Probably the easiest way would be to hide small, pre-written sheets in your mouth (maybe just "yes" and "no") and find a way to rotate them. Or just a small, two-sided plate that you can easily flip with your tongue.
Reward: I want a sample so I can joke around with it at the next party