If you like Amiga Workbench, you can always try MorphOS. But that is not a straightforward process to get into. And it's not free software...
If it's an earlier model, they make great 'retro' games and software machines. Install 10.4 Tiger, set up the Classic (OS 9 environment). That gives you a decent chance of running stuff from System 6-ish (late 80s) up to mid 2000s. Not like Macs had a lot of games or unique software, but it can be cool for the heck of it.
Also, with their FireWire ports, it can be handy if you still have old DV or HDV camcorders, pretty straightforward to capture old tapes. The later models are better for this purpose, more processing power and better graphics cards.
If neither of those use cases appeals to you or sounds like an applicable use case, the G5 towers are the best to use for ATX case mods. But you did mention you didn't see a point in that. Therefore... Recycle it or pass it on to someone who is interested in tinkering with it. After 20 years, just like a lot of computers from that era, they don't have alot of justified use cases... And that's okay!