You could make it a backup server. Just turn it on when you want to make a backup. 
I could do that by putting the hard drives on external enclosures. It'd me more feasible, smaller than a mid tower, no need for extra hardware, and I could put them away in a drawer when I'm done 
But, you asked what you could do with hardware you already own.
I'll remember this.
I'll remember this.
Yea but might as well suggest I use the fans for moving air through my room, man