[Mongoose]

To elaborate on the idea even more, you could try the following: If your robot has ball pick-up capability (not everybody has that), you could scatter balls around an arena and have people try and pick as many of them up and put them in a goal in, let's say, 5 minutes (for $5 or so). Then you could give away prizes (stale lollipops!) for picking up certain numbers of balls. And if you don't have pickup capability, I guess you could put together an arm and mount it on the robot or else have the object of the game to be to nudge all the balls in some corner/area of the arena instead of picking them up. Just more ideas.