Building and running a multiplayer game on one might be cool. Websocket is nice for making simpler real-time games for the browser. Godot also has multiplayer networking support but I've never tried it before.

You do have to open up the Pi to the public Internet though to get any people to play. I use Tailscale Funnel but there are probably also other tunneling solutions

A. run Linux on it

1. run a Lemmy instance

/s

I run a Jellyfin server on my SBC. Mine happens to be a Le Potato

If you use signal, switch to molly unifiedpush and host mollysocket and ntfy, and use unified push

jellyfin or navidrome

attach a hard drive and use borg backup to back stuff up to it