When I was a kid in the early 1980s I went to work with my dad in the summer pretty often. I'd hang out in his shop while he made electronic equipment for the University. It was a playground of bins containing wires, resistors, potentiometers, rheostats, circuit boards of unknown origin, and a host of other unexplainable things.
I am now 39 - about the same age my dad was back then. I have a computer in my pocket that blows away anything from 1981 and I'm still interested in the stuff from his shop.
Here's some stuff I've made, repaired, and modified:
Beer fermentation temperature monitor/controller
Arduino based fermentation temperature monitor and controller prototype. Features custom jacks for easy addition and removal of multiple digital temp sensors. Includes the ability to scroll between them using the LCD interface on the front panel. Includes a wifi shield for sending temperature data to (and receiving settings from) Raspberry Pi. The solid state relay allows for the possibility of controlling heating/cooling elements to keep your beer fermentation at the temperature you select - regardless of the air temperature. I wrote all custom code for the Arduino and Raspberry Pi. The cigar box is easy to work with - you don't need fancy tools or a wood shop.
Custom sensor input jacks