I guess a decorative piece. They are really cool and unique with their lampshade design and I would love to have one on my bedshelf on display! So I would just set it up somewhere where it looks good. You could always part it out if you don't want to put it on display.
Well I don't know how much a PSU would cost, but if it's less than buying a used G4 off of eBay or CL and then taking that one, then it might not be worth it.
Most of the time, PSU failures are a bad cap. This isn't hard or expensive to replace. No matter what you do, don't recycle it. I would keep it for decoration or simply buy a cheap PSU for it. Give the MacRumors Marketplace a shot if you want some dough for it.