Thanks for the replies! Happy that it was useful for some!
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.
Yes, I definitely agree on that. I just quicly sketched them out in fritzing... :
I'll make better ones very soon.
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:
This is very nice idea! Even the solenoid thing would be great by itself. But utsing LOGO would be really great! One of my first steps to programming besides vic-20 and BASIC, very nostalgic idea