I've been working for a while trying to figure out a reliable way to use the HC-SR04 ultrasonic proximity detector with this robot. Thanks to the good people down under at Nodebots Australia, I think we've got it working great!
When my hand is away, the robot drives forward. When my hand is in front of the sensor, the robot turns to the right to avoid it, then continues to move forward when my hand (the obstacle) is gone. Check it out in this little video: