Stuff I tried out, but not as useful as on a x86-compatible machine:
Docker: There's still too few images built for armv7 support. Some images I tested and are usable:
n8n.io: Although the Docker image for n8n.io does not have armv7 support, you can still easily install this via good old npm.
Home Assistant: Everything I do currently is merely controlling a smartbulb, and HomeBridge suffices for this purpose, so I felt no need to migrate over.
I used to enjoy connecting to my Raspberry Pi via VNC. Recently, however, a GUI started to mean less to me than it did a couple of years ago. Nowadays, I only spin up a VNC server on the Pi manually when I feel like to.
During my Raspberry Pi 2 era, I had one of those 3.5-inch TFT-panel touchscreens that come with a transparent case. It was not until I had started assembling did I realize that those units need a custom OS image. The customized image was based on an outdated version of Raspbian. Lacking experience with Linux drivers, I was unable to "extract" the driver and install it to a fresh install of the OS. Therefore I considered it too much a hassle and didn't bother using the touchscreen at all.
I was also eyeing on a robotic arm or building a self-driving car. After browsing through some sub-$300 options, I deemed it too pricey a hobby to take on for so little practical use.