Sonar scanner
360° sonar scanners have been done before but the ones I've seen make the whole vehicle rotate - this is not an elegant solution. Can you design a sonar scanner using the ultrasonic sensor that can rotate continuously without requiring that the whole Lego vehicle rotate? The difficulty here is allowing for a continuous rotation without the sensor cable getting twisted. The closest I've seen is this one, but it still requires that the big, heavy brick rotate with the sensor - I think it should be possible to avoid that.