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:
from adafruit_hcsr04 import HCSR04
sonar = HCSR04(board.D2, board.D3)
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