The new HackSpace Magazine Issue 27 highlights using the HC-SR04 distance sensor with a microcontroller. Using the Adafruit Grand Central, the readings can be done in Arduino or much easier in CircuitPython.

Ultrasonic distance sensors work in the same way that sonar works. A high-pitched pulse of sound is sent out, it echoes off nearby objects, and the sensor listens for its return. The longer the time between pulse and echo, the further away the object is. The common HC-SR04 sensor is widely available, and often costs just a pound or two. (Ed: Under $5 US)

The code to read the sensor in CircuitPython is easy:

import board
import time
from adafruit_hcsr04 import HCSR04
sonar = HCSR04(board.D2, board.D3)
time.sleep(2)
try:
    while True:
        print(sonar.distance)
        time.sleep(0.5)
except KeyboardInterrupt:
    pass
sonar.deinit()

You can read the full tutorial in Issue 27 of HackSpace Magazine (PDF pages 110 to 111).