Your idea of internet filtering isn't too bad on such a machine. A base install of something like Slack or Debian, with a basic Squid + SquidGuard/DansGuardian wouldn't hurt your Internet pipe all that much and should pretty decent, depending on how much net traffic you have. With internet filtering and the other suggestions of Seti/Folding/etc.@home, it depends more on how much memory you have.
Alternatively, stick a basic KDE setup on and run the Matrix screensaver. I left that running for a year in a server rack and always looked impressive - I told people it monitored the whole network. Can't remember what it actually did, but it wasn't much!