I made this robot by modifying the tracked robot in the included 
instructions. It had 2 motors, 3:1 drive ratio, and 2 touch sensors. 
I solved the problem of robots getting stuck in corners (bump right 
bumper, back up, turn left, go forward, bump left bumper, repeat 
until batteries die). I added a timer block to the watch blocks for 
the touch sensors; when both bumpers were triggered within a short time, 
it would back up, do a 180, and go the other direction. 

After this project, I experimented with the idea of using the RCX's 
IR transmitter and the light sensor together as a sort of sonar. It 
worked, but it was a HUGE battery hog! Keep in mind this was the RCX, 
not the NXT, which comes with an ultrasonic proximity sensor.