Idea's for what to do with a Spare PC

I know this has been asked before, but instead of reviving and old thread, figured I'll just ask.

Anyway I got my hands on a few PC parts and was able to build another PC, I'd like to keep it as my spare just in case my primary goes down for whatever reason, I just went out to buy a cheap case for it. But I hate having it sit there and do nothing or waste power on the login screen just to make me feel better or sit under my desk collecting dust.

I'm just looking for some cool ideas I can possibly do with the thing to make me feel like its useless. And yes I've googled around, and some ideas were good but didn't seem practical. This spare computer I was leaning towards setting it up as a media server and throwing on some Videos and maybe a few files for the other computers around the house, but I really dislike Windows networking as I always seem to loose a computer on the network or it sees it but fails to access any given computer, I'm sure its the combination of my router and 8 port switch.

Anyway, the PC specs are:

OS: Windows 10 x64
CPU: Intel Pentium G3258 4.2ghz
RAM: 16GB Corsair Vengence Pro 1400mhz (sticks are rated for 2400mhz but limited by the board)
MOBO: Gigabyte GA-H81M-H
VIDEO: GTX580 (Card I was testing and it's faulty, but I needed a DVI port so I left it in)
PSU: Corsair HX850 (I got this PSU for $34.99 USD at Micro Center, its recertified from Corsair, Heck of a deal)
Storage: 120GB SSD / 500GB 2.5 5400rpm HDD
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If you dont want to sell it you could drop the GPU and use it as an NAS, (or media server like you said). Low power integrated graphics would be ideal for something like that.
Other options are a streaming PC, but that is very specific and it may not be up to the task.