One great alternative that will breathe new life into your Mac hardware (regardless of age): Install Linux on it. When you've finished this guide, you'll have set up a dual-boot Mac and Linux system that will allow you to switch between operating systems anytime—meaning it's a low risk way of trying out Linux to see if you might prefer making the full switch on your Mac. I think you might just like what you find.
Ready to give it a try? Pack your bags, we're defecting to Linux.