> I accidentally wound up managing a cafe about 25 hours a week for free, was that not work?

As long as the cafe pays taxes, follows the law like any other business, and claims you as an employee, sounds like work to me.

You could also be part of a nonprofit that holds mini-concerts in nursing homes, provided that all the appropriate tax paperwork gets filed.

I worked as a volunteer, not an employee. I also could've been volunteering looking after plants, or sitting around doing nothing in particular except chatting with people, and the organisation would've likely signed off on it. I also could've been volunteering as the one person who knew how to manage the sound system and turned up for two hours a week but was the reason we had any customers at all. There's no reason some random company is trustworthy, or some arbitrary amount of time is valued specifically.

Would self-employed work not count under your system? What if I was doing something with the hope that it will make money in future? What if it's not, in fact, intended to make money at all, but would be non-profit work? If I set up a company that would sign off on me and my friends, would that change anything?

Managing a cafe for free because you want to help the owners or you want to keep the place around but now it won't be able to sustain an employee etc: free work.

Managing a cafe in Cafe Simulator 3000 because you want entertainment: not work (but these games often do a good job emulating the monotony of work in the long run).

The lines get blurry when you're managing an actual cafe because you're bored and want entertainment.

Hanging around a cafe because your friend works there: not work, even though some people sometimes pay other people just to hang around in a cafe.

So how do you define "work" in a way that covers one but not the other in a way that reliably can't be gamed on a country-level scale without spending utterly ridiculous amounts of money on it?