Inspired by Adafruit's @johnedgarpark, I created a set of screaming Halloween cauldrons for use in a preschool trunk or treat event. Using four Sharp proximity sensors, four Trinket M0s, an AudioFX board, a small CircuitPython script, and a speaker I was able to get a few frightened jumps out of the kids.
This same concept could be repurposed to play a Christmas jingle when people approach your front door, an alarm when your annoying sibling is invading your room, or a tone when a customer enters a store.