Usually the first robot you make when you get the NXT is the wheelbase with a bumper, you know, make it run into a wall, turn back and repeat.
To really get my point about saving sensors across, I made a robot like that without any sensors.
It works by turning the motors in regulated mode(constant speed, varying power) and measuring the actual power applied. If the robot runs into a wall, the firmware will apply extra power to the motors to keep them turning. With some tweaking, you can even detect which wheel hit the wall.