A wall graffiti plotter - works like a small robotic window-cleaning dolly with a spray paint nozzle. It is controlled basically like a x-y plotter, where one dimension is movement along the top rail and the other dimension the length of the cable to hang down. Extra points for a system to compensate for movements of the basket with accelerometers or gyroscopes.
To prevent problems with authorities, use chalk-water colours which will wash off with the next rain or a gardening hose.
Is that showcase enough?
Korman