Just as a thought, what about some sort of tricky-to-score-in goal?
Consider the field in Dave's infamous curling teaser:
http://www.team116.org/Images/CurlingBots.jpg
Now add a bar that scoring objects can go under (but robots can't) just before the banked curves, and delete the middle row, putting some sort of goal basket in its place. Robots get a small amount of points for getting it past the bar, but the big points are earned by getting the objects around the bend and into the goal.