Arduino plus Ultrasonic Sensor hardware:

The Barobo Starter Kit (Arduino-Uno-Compatible) includes an ultrasonic distance sensor among its items (the one that looks like two eyes):

The ultrasonic distance sensor is also available as part of the Robot Sensor Pack, or it may be purchased individually.

Connecting the ultrasonic sensor to the Arduino:

The circuit diagram for connecting the ultrasonic sensor to the Arduino board is:

The image below provides another view of the connections. The VCC pin of the sensor is connected to the 5V pin in the Power section of the Arduino, and the Gnd pin of the sensor is connected to one of the GND pins in the same section. The sensor's Trig pin is connected to digital pin 9, and the Echo pin is connected to digital pin 8.