Are you geek enough ?
Try this QR Code door hanger and make your room mates wonder.,
or use it when you want to keep a secret message away from your granny ...

This doorhanger was created with my python-enabled openscad fork. It has all the power of python integrated, so no extra step or intermediate scad code needed.

Print the STL with white filament then use a small brush to carefully paint the surface of the pixels with black varnish. It works incredible well.
Customize your secret message. check my openscad site at

http://www.guenther-sohler.net/openscad