You may have this the wrong way round - you've got a solution in search of a problem. 'I want to use this computer what is it good at?' If you had a use-case for it, wouldn't you be using it already? What keeps you using your Mac mini instead for general purpose computing stuff?
What do you like doing on computers? That way folks responding can tailor their suggestions. If I told you the 400 exposes the GPIO pins and you can pick up a breadboard adapter for it and interact with physical components via python, would that sound interesting to you or mean anything to you? Have you searched the internet for "raspberry pi projects" and seen anything that seems interesting to you? Do you just need help whittling options down?
I have a handful of pis, none in the 400 case though. One runs pi-hole as an ad/tracker blocker for my network, one is in a little case with a 7" screen and I use it for emulation, one is a general purpose computer, one is hooked up to my a/v receiver as an mpd player (though it's been a while since I've used that).
Many of these have the same underlying external cause of money no longer being free so they can't keep the merry-go-round going for their debt and/or their investors are no longer content to just pour more money into the engine.