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.
I went to a vocational school my last year of high school... many years ago. I was in the Micro Computer Tech class and they were always looking for donated equipment. We got alot of older stuff, but were excited when something within 10 years came in the door. We got to build some 8088's. Now THAT will make you thankful for a modern BIOS.