I turned an unused parking spot near a hospital into steady monthly income by listing it on a peer-to-peer parking marketplace.
Small self-storage facilities sometimes sell individual units. Owning one and renting it produces boring, reliable cash flow.
Rent long, sublet short (with landlord permission). Modest but real cashflow per unit.
Twelve units, one pickup truck, one Saturday route. Boring, ugly, and prints money once contractors know your number.
Billboard operators discount unsold weeks to almost nothing. Buy at the bottom, resell to local businesses at retail. Idea — not yet tested end-to-end.