Asklemmy

You could host an instance of a fediverse platform (like Lemmy!) if you're interested, and up for the responsibility of moderating and maintaining the community.

An easier thing that requires less moderation might be to host a privacy oriented "alternative client" for large platforms, such as Invidious for Youtube, Nitter for Twitter, or Teddit for Reddit. They provide users a way of accessing those non-privacy-respecting services while giving minimal data to them.

You could also host your own VPN or DNS/PiHole server and be pretty confident that the server isn't collecting data you don't want it to (though your VPS is itself still a concern in that department).

Running BOINC or Folding@Home will donate your server's computing power to science! Both run calculations for research projects on the local machine (serious ones too, like COVID or cancer research, or crunching the math on the results of particle accelerator experiments).

Finally, if you have an open source project that you want to support, seeding their release torrents and/or providing a mirror server for their package manager or general file store is an easy way to help them out!