This idea isn’t passive in the sense you have to do no work. But it’s a hobby I’m doing anyway, so making a little income from it requires little to no extra work that I’m already putting in for the hobby:
I started buying collections of baseball cards and selling them individually/in smaller lots. When you buy a large collection, the price per card is a big discount. You often don’t need to sell every card individually to make a profit either (while still selling the cards at fair market values; don’t deceit people…). I imagine there are lots of other collecting hobbies that this form of “passive” income can be applied to as well.