Ultrasonic HC-SR04 echo and trig pins
To any 2 free IO pins.

To quote sparkfuns datasheet:


  (1) Using IO trigger for at least 10us high level s ignal,
  (2) The Module automatically sends eight 40 kHz and  detect whether there is a pulse signal back.
  (3) IF the signal back, through high level , time of high output IO duration is  the time from sending ultrasonic to returning.
  Test distance = (high level time×velocity of sound (340M/S) / 2,


In other words you send a short pulse to trigger pin and then wait for the echo pulse and measure how long it was high. Divide the number of microseconds it was high by 58 and you have the distance in centimetres ( use 148 for inches)