UV laser pens leave a luminescent trail when shone on glow-in-the-dark material. In this project a 405nm laser beam is steered by a pair of Arduino controlled mirrors leaving a sine wave based trace on a sheet of luminous vinyl. You can see it in action here https://www.youtube.com/watch?v=MUtxg7VziGE and there are some more details on my notes blog here
http://notes.robives.com/laser-galvo/ There are several things to try next:
Writing code to create a raster scan to make a rudimentary TV style screen
Making an oscilloscope or ECG
Using stepper motors rather than servos.