Diy guitar effects often cost more (once the casing is included) than the commercial versions. Since they, by definition, exist in order to distort or change the signal, they aren't good training for hifi--they're pretty much an end in themselves.
As far as hifi electronics go, the easiest project would be a simple preamp or amplifier. One of Nelson Pass's preamps (www.passdiy.com, the diy arm of Pass Labs) will be about as simple as you could ask for, and fairly inexpensive. The amps aren't much more complicated, but will cost a little more due to needing larger transformers and caps.