Other than Friendica, Mastodon, Matrix, PeerTube and PieFed, what’s worth running

Maybe Nextcloud (not only for storage, but also calendar, video conferences, office, when combined with Collabora,…)? Also Immich (basically Google Photos) comes to mind. Your own instance of SearXNG.

Any kind of ToDo-list, Kanban board, …?

A ticketing system?

A Wiki to host your documentation? Note, that you may want to access it if the server fails, so…

Some stack of components around Grafana or such to visualize some data? Since you mentioned Hetzner, I’m guessing you’re from Germany. You could build a small container, ingest the gas prices that the gas station are required to publish and build a dashboard for the gas prices in your area? (Hint, here’s an API licensed under Creative Commons - creativecommons.tankerkoenig.de)