My work has a really old phone system which I became the admin of. I discovered it had an audio input for Music-On-Hold, so I set up a spare RPi Zero that I had as a music player so that we'd have hold music. Low effort but wonderful improvement for work. Every now and then I go back and tinker with it to improve it a bit. (Next step is to make it controllable via a web interface.)
I have a RPi 3B+ that I use for some emulation, though I hardly ever play with it. Setting it up was plenty interesting, though.
And I have a Zero and a Zero W that I use for random tinkering/testing, both semi-permanently attached to a breadboard for ease of use.
(I've got a big list of projects I'd like to try or develop, but the above are the only things I've done so far.)