RCX robotic car
How it works: After the start button is pressed the "car" starts moving forward until it senses an item blocking its way. Then it goes backwards for 1-2 inches, performs a 90 degree right turn and starts moving forward until it gets blocked by another item.
The sensor part was the most interesting. There is a color sensor that informs the RCX if the color is different than the air, in order to stop moving forward. In case this sensor fails there is a touch sensor that detects easily a collision.