Ask HN: What passive income investments exist to supplement your salary?

I was reading this article (https://themdpreneur.com/purchase-future-cash-stop-buying-more-stuff/) on HN the other day and it talks about a hypothetical person who buys a condo and rents it out, or a vending machine and has it serviced, and a website that brings in income.

I come from a fairly traditional investing background of purchasing equities or ETFs as my primary investments, and am interested in learning what else is out there in a non-traditional sense like the above?

What other kind of passive income or cash flow helping investments are out there?