Fix up their computers for free, and install zombie software that lets you use their computer whenever you want for any purpose, e.g. posting & tweeting, emails, cryptocurrency mining, distributed computing, etc.
On second thought, that's a bad idea.
I got nothing that I'd actually consider doing there or think that would be worthwhile.
The big problem as I can see it is that it will be very hard to reach that audience. You're more likely to get more business by starting a blog for beginner computer problems, do some YouTube videos, build all that up, then offer classes or do the speaking circuit trying to get corporate education gigs, etc. Dunno. If they have crappy hardware, they probably have crappy budgets. That's going to make things rough.