I found an old Western Electric rotary phone in my attic. It's really an amazing piece of hardware. It seems to work fine several decades after its construction despite whatever abuse it took before ending up in my attic. However, I don't currently need a rotary phone.
So I decided to put a Raspberry Pi inside it. I decided to make it an MP3 player that's simple and robust for my daughter, but I guess it can actually be a general-purpose (if inconvenient) computer.
The pictures show the finished product. My goal was to leave the phone as intact as possible (ars est celare artem), so the main sign anything has changed is that the port on the back is now micro USB rather than a modular phone jack. I also added a switch to power down the Pi in the handle below the hook, but I wish I hadn't -- the handle was really comfortable for carrying the phone and the switch gets in the way. So these instructions don't show that part. Since I wanted to add as little as possible, I put the speakers in the headset.
Here are the things I used.
Raspberry Pi I used a model B but it should be possible to modify the instructions for another model instead