We've got this cheap egg cooker that plays the loudest, most terrifying alarm you've ever heard when it's done cooking. My wife requested that I make it "less aggressive" so I hacked it into the most gentle egg cooker of ALL TIME

(warning: probably loud)

These are such beautifully simple machines. It's got a heating element, a fuse and an alarm. when the water evaporates, the plate gets hot enough to open the fuse. The power is then rerouted through the alarm. So you control cooking length simply by adding more/less water!
All I did was replace the alarm (which took 120V AC directly) with a little AC-DC module (basically what's inside a phone charger) and so it sends out 5V DC when its done cooking. I'm knocking it down to 3.3 and hooking it up directly to the the music box.
Canon in D Chorus: Mechanical Music Box Breakout by Questwise Ventures on Tindie

Unique mechanical music box board: plays 'Canon in D' or CUSTOMIZE your own song, adjusts tempo with PWM and offers interactive playback

Tindie