Almost always when I'm happy about something I've finished/completed/done, it's just a small thing that nevertheless provides satisfaction.
Like getting rid of the really really ugly and impractical door knobs in my house and replacing them with levers. (Whoever invented door knobs never had two hands full.)
Or receiving my new Japanese pull saw in the mail and immediately using it to fix a small blunder I made a few years ago. My sister recommended a pull saw, and I really like it because it cuts much nicer than all the hand saws I've owned so far. (Naturally the local hardware shops didn't have anything but the usual crap, so I bought it from this guy in Sydney.)
Or retrofitting all the cabinet doors in my place with soft-closing dampers (Airtic). Or diagnosing and fixing an annoying electrical problem with the car's power window. Or lubricating my bicycle until is whirrs. Or...