My most successful side project from last year is this climbing community that I built for my wife:
We live in Fontainebleau, France and have arranged our life around the bouldering here, but she had always struggled to find good projects because the climbers who travel here tend to be mostly gym-strong men who power and lank their way through the problems here that suit that style. But there's a ton of other stuff that needs good technique and benefits from being able to bear down on small holds. Women are good at that sort of thing, but there was never a way to find information on those boulder problems.
It made a perfect "Side Project" because the core idea was buildable in a single day, leveraging the same framework I'd used for S3stat, Twiddla, Unwaffle, and the other "Real Business" projects I'd been doing. I was able to avoid silly Javascript build systems and other time-sink pitfalls that normally plague everybody's side projects (presumably because they seem fun and you're trying to learn stuff in addition to building something). The only real "reactive" stuff it needed was in a little "Project Finder" that lets you adjust sliders for grade, popularity, style, climber height, etc. and would benefit from having the list of climbs update in real time:
https://bettybeta.com/bouldering/projects
... and that was just a few hundred lines of vue.js, again minus any build system.
I still stick new stuff in regularly (like the "remoteness" filter so that you can find boulders deep in the woods where no plague-carrying-zombies will find you), but since it's built on a polished and time-tested architecture, I can get an awful lot done in a couple hours, so it's just a matter of disappearing off to the office on a rainy saturday morning, then coming back and announcing a few improvements.
Fun stuff.