I don't know how complicated you want it, but you could do something like, your robot has to be able to find its way to the center of the field from any location. The robot would start in a known position, and then be driven around randomly. A button could then be pressed and the robot would have to locate the center of the field. You could include obstacles in known locations, but to make it complicated, the only sensors that could be used are encoders.
Just an idea.
