Interesting robot, something I have long thought about playing with, but have never gotten around to it. A suggestion:
I would re-draw your schematic and the breadboard representation to avoid the "rats-nets" layouts you currently have (especially the schematic; the breadboard layout, I realize, may not be as straightforward, but it can be done better). This will just make it easier for your visitors to understand what is going on.
If you made it so that your robot could raise and lower the pen (maybe with a solenoid - you would probably also need to add encoders to the wheels to track distance), you could re-create a classic LOGO turtle device; write a simple interpreter for the Ardunio for LOGO, and LOGO commands/programs could be sent over the serial link for the turtle to execute - see these wikipedia articles for a start:
Finally - I think the "work" that the robots produced was actually quite beautiful, in a "Jackson Pollock" sort-of way...
