In the classic computer game Pong, the angle the ball bounces off the paddle depends on where on the paddle the ball hits. This could be nicely illustrated by making the paddle shaped like a semicircle.
Several difficulties: the bounce point is no longer strictly on the back wall -- the plane of a flat paddle; it may be separated from the back wall by the radius of the paddle. Should the momentum of the paddle become transferred to the ball? If so, how much?