I run a robotics club at the school where I teach, we're building a small rover based on this chassis:
This, a breadboard and some arduino parts with a motor driver chip are all you need, but we're putting an IR distance sensor on it for some collision control. It's not that hard to breadboard it all together or make it more permanent if you like. I'm getting a lot of parts from oomlout (uk site), whos service I have to say has been amazing.
For introduction to the arduino, programming and electronics they are messing with led's and rgb leds for the moment.