I've only ever built a "baby's first fuzz pedal" which is one of the easiest. You can find tons of schematics hereand on the website hack-a-day. My Banjo is actually pretty nice, when I say I'll never learn it, I mostly mean learn it as well as I can play guitar. I can pick some notes and diddle around on it, but nothing special, but I don't think I can let go of it too soon since one of band uses it. I keep it as an "if you provide it, they can't say no" kind of deal. I can play the Mandolin like a beast though. If you want to pick up a nice starter Banjo I'd recommend a Deering's Good Times. By starter I don't mean dingy, I know a lot of folk who play these all the time.
jonaswildman, have you ever built any of your own gear? Janabutts, thanks for the links. That Deering's Good Times is a pretty banjo and for $400 it's definitely not too expensive. That said, I have to buy a violin next. Actually two violins, one for my 2.5 year old daughter and one for me. Actually, I'll likely rent hers and buy mine. The mandolin is such a lovely instrument to play. I probably play the mandolin like you do the banjo. I know about 6 chords and can fake my way towards almost sounding like I know what I'm doing. I have zero technique though. I play it like a rhythm guitar and have barely any picking skills. They sure are fun to play though, aren't they?
The violin's a heap of fun, and contrary to what most people seem to think, it isn't especially difficult to pick up. I've found so far that the fingering is similar enough to the other string instruments I play for it to be easy to pick up. The bow is the trickiest so far, but it's getting there.
Ive never have had the urge to build anything related to music. That side of music doesn't interest me at all. I thought it was cool that Elliot Bergman made his own Kalimbas (http://m.youtube.com/watch?v=1YwFw4ZH8F0) and sold them at Nomo shows and I am intrigued that The Books make their own stuff though.
I keep this 4 string guitar in my office to pick on when I get the hankering: it's a lot of fun.
Don't be jealous, I work from home :-) I could keep a drum set in my office and no one could care less. In fact, I will often sneak downstairs in to my studio mid-day to play the drums. It's really nice. I'll never work in an office ever again. Not ever.