For me, the best small side projects involve some kind of porting (between protocols, OSs or CPUs), contribution of a fix for a visible but mysterious bug, or revival of codebases in bad shape, especially when they're unfamiliar and written by others. You can learn a lot from a software engineering culture that's different from yours or small, feasible projects that allow you to understand the bits and bytes of something unfamiliar in a fun and useful way. These projects are great to develop flexibility and general problem solving intuition. And getting familiar with the 20% of an idea, a protocol, a programming language, etc' that's used 80% of the time in real-world use cases, is often enough.

https://github.com/dimkr/gplaces (Gopher -> Gemini port)

https://github.com/dimkr/paho.mqtt.embedded-c (revival of an abandoned library, WebSocket+TLS+Windows+Meson porting sub-projects)

https://github.com/dimkr/loksh (OpenBSD -> Linux)

https://github.com/dimkr/locwm (OpenBSD -> Linux)

https://github.com/puppylinux-woof-CE/woof-CE/projects/1 (a bunch of GTK+ 2/X11/X.Org -> GTK+ 3/Wayland/wlroots porting sub-projects)

https://github.com/dimkr/gtk (x86_64/ARM64 port of a 90's library)