Mount an ultrasonic range finder on the front, the board itself costs only a few dollars. Mount on a servo, the servo rotates back and fourth (max 180). If any obstacle comes within a certain range, the robot will turn the wheels. The hard part will be coordinating the sensor with the servo, and then creating the logic which controls the wheels. Nevertheless, its been done before and we can build it too.
Video: Arduino Obstacle Avoidance Robot
