For many years now, I have been periodically scanning eBay of used airplane avionics, or aviation instrumentation. Once or twice a year I'll see something cheap enough (under $50) and fascinating enough that I'll buy. In the past I've restored or made operable all sorts of unusual little devices and learned a lot about the way complex instruments like this used to be designed.
The 747-200 CDU is my most ambitious project to date. This device is basically an early computer, probably one of the only computers actually onboard an airplane when it was designed. It was a predecessor of the now-common flight management system, which is basically a overall control system tying together the autopilot, auto-throttle, position, altitude and course management systems together.
This CDU is basically a "head end" terminal to a complex computer that lived in the avionics bay of a 747. This computer itself is long-gone, but for my purposes it doesn't matter.