Users on sites such as Thingiverse never cease to amaze me. For example, Thingiverse user Johannes has created a 3D printed clock that writes the time with a whiteboard marker on a whiteboard and then wipes it out.
The Plotclock is essentially a simple robot and can be built with a 3D printer or a laser cutter. It features an Arduino Uno microcontroller, three servos, and an assortment of nuts and bolts. The three servos control the pen and the two robotic arms that move the pen.
While this is hopelessly impractical and I’m sure we won’t be walking round with these in our pockets or our wrists, it would be a great project for school kids.