Install a word processor on it and donate it if you don't want to keep it, if you do, I don't know what to tell you. It's hard to come up with uses for a computer you don't need or use. Especially when you have newer, more functional ones at your disposal. If it were a windows machine of that age we were dealing with, the answer would be simple, it could work with anything a PC from 2014 can with no workarounds. The same can't be said for PPC sadly. Hell, right now I'm on a PC from 2006 (just something I set up today for ***** and giggles) and it's running Windows 7, Office 2013, Firefox 29, playing YouTube all without a hiccup. I can put in a USB3.0 card, a blu ray drive and any other peripheral I wanted and it would work right out of the box.
Best of luck in finding what to use your G5 for. Worst comes to worst it can be a room heater for next winter. If you have kids or a guest room you could set it up in there. Or put it in a workshop or garage to use there assuming you have or can get internet connectivity to those areas.