I turned an unused parking spot near a hospital into steady monthly income by listing it on a peer-to-peer parking marketplace.
An idea I keep coming back to: use modern translation tools + human editing to bring forgotten public-domain books to English readers.
Small self-storage facilities sometimes sell individual units. Owning one and renting it produces boring, reliable cash flow.
One page. One skill. Pay-what-you-want with a suggested price. Traffic comes from search and word of mouth.
One well-designed intake and session-notes template, sold on Gumroad, quietly earns while you sleep.
Contracts, meetings, hard hats, spreadsheets. Boring photos sell every month, forever.
Small footprint, cash business, low competition against Coke and Pepsi.