Oh man it's been a while, but I used to be super interested in getting an Air Quality Monitor going. There's definitely ones that can seed their data to public databases.
Things like Freenet or ipfs where your node doesn't provide just networking but also storage.
Could be too obvious to mention here but a fediverse instance of some sort, lots of different software you can run besides the usual suspects like Mastodon or Lemmy/Kbin ranging from the more niche microblog platforms like Misskey forks (writing this on a Sharkey instance) to things like Bookwyrm for tracking your reading habits and connecting with other book lovers.
Distributed computing like distributed.net or Folding at Home.
I host wikis for local advocacy work. I plan to host other static sites, and I'm building a tool to get better transit schedules in a simple webpage. Hosting Mobilizon is on my list to do as well.
Not that I do it myself, but I belive with an SDR dongle you could contribute to both aircraft tracking and naval tracking. Big communities out there for this at least.
It was an open community and then the person who ran the server sold it to a corporation for over a million dollars. I believe the community strongly dislikes the person and spun up their own website and server https://globe.airplanes.live/
If you decide to set up an SDR for ADS-B, you might want to consider setting up a WebSDR with something like OpenWebRX. This would let people listen to all the signals in the bandwidth that you set.
If you're interested, receiverbook.de is a list of most WebSDRs.