Not much you can do unless you have kids and want to make a little PC for them to tinker on. Or niece/nephew, or something like that. Or even a grandparent. You can always turn it into a file server, but that's just a PC sitting around always on sucking power. I'd either part it out or make it a complete build and reinstall the OS on it and sell on Craigslist. I've sold one or two systems on Craigslist pretty cheap. As long as it's a fresh OS and cosmetically it looks nice, people are usually willing to buy pretty quick.
I've sold some old systems or the main components like mobo,cpu,mem and sometimes case as a barebone system. I've also once given an old PC to an older woman I worked with who was on a limited budget so her and her husband had internet and email. I fixed one up once and bought a few parts for my sister and brother-in-law as a gift since they needed a new PC and just got married. I've also helped put together a PC for my grandmother a long time ago so she had email. Other than that I usually just sell components or place them in my buddies basement on the shelves of shame.