Once I wrote a Forth for OS/2 in Assembler out of spite... because it was Impossible to do so, you were supposed to only be able to write for OS/2 in C (which I hated). Brian Mathewson came along and wrote a manual for it after I open sourced it. Much fun was had by all.
One day, I was working away, and heard my co-worker talking to the owner of a light bulb company... and he was describing the need to "season" halogen lamps, and was wondering how to keep the filament from sagging while it was heated the very first time. I suggested they levitate the filament, and set up a demo with a thin wire the next day. I learned a lot about the manufacture of bulbs, and helped light the world. (The technology wasn't invented by me, GE had done something similar decades ago, but how was a trade secret)
All of these were funded by coding.
Work doesn't have to be fulfilling. It's cool if it is (I'd much rather get paid $5000 to perform at the comedy club rather than two free beer tickets ;-)) but don't worry if it's not.
Tools at work that help people do their job much more efficiently.
Changing my psychology to be a happier person.
Yes, making an impact on others is very rewarding.
Making things for myself is nice, of course, in fact the pleasure of making something that I myself really like can be greater than the pleasure of making something for someone else. When I say "pleasure" I mean something like a sensation in the body, the stomach.
But things can be greatly rewarding without involving much pleasure. This can often involve a sensation of warmth in the heart.