A PowerMac G4 will run the latest OpenBSD. https://www.openbsd.org/macppc.html I run it on one of my iMac G4's (and on many other non-PPC machines). It occurs to me that if you replace the HDD with an SSD it would also be quite a lot faster.
If you don't want to use it, toss it on Craigslist for sure (if you have a local Craigslist site). Depending on the specs and your location I'd expect around $50-75 CAD (basing this on Vancouver Canada area and habitually searching the words "macintosh", "powermac", "powerpc" etc. literally every day :) )
I bought a g4 laptop for vintage computing, but once the initial excitement retired I kept it on bookshelf. I don't regret buying it because it only cost 50 bucks including postal charge and even the battery still works.
So yeah I agree with a fellow commentor that if you don't know what to do maybe find someone who can pay premium and sell it.
Might want to keep the battery charged every so often or it will die completely. My iBook G4 batteries from ~2004 still hold plenty of charge, surprisingly enough! I've had far newer laptop batteries die completely because I forgot to recharge them... :(
Nice, I used to use one of those at work! That was a weapon of mass creation back in the day. I don't know what counts as cool to you, but to me just using the thing as-outfitted seems pretty cool :D
A kindred spirit. I have Arch running on a 2008 iMac. It is almost embarrassing how much I use it. It is my main video conferencing spot as I like the big 4:3 screen. I was playing Diablo ( DevilitionX ) on it earlier today.
A quick look online suggests people value it around 2-3x as much as an i5 machine with 8GB RAM.