I have a unused RPi4 (the 8Gig one) running DietPi.
I did use it as a playground but ever since I am renting a Hetzner machine for (playground) stuff that I want web accessible, I don't have particular use for the Pi.
I am currently running (outdated) Home Assistant on it but there isn't much I can connect it with (yet, getting the flashable/compatible ikea smart lightning zigbee? bridge thingy is on my bucket list).
Obviously I do have a pihole there.
Shoot me any other ideas I could run there.
Some kind of monitoring of my rented infra would be cool (I already have uptime kuma on the dedi hetzner box). One idea I had was if there are some OSS security scanning "daemons" I could use on to monitor my other infra.
Immich! It's an amazing self hosted Google Photos replacement.
Zigbee definitely fun with HomeAssistant. I have an SLZB-06M adapter which has PoE (important for me) and is a fairly "open" product (don't need to jump through hoops to flash firmware). I read somewhere that it may offer Thread support at some point but wouldn't count on that.