I sell/invest in Pokemon/Magic the Gathering/Yugioh Cards. It's some work to keep up to date with prices, managing the online store / in person matters. But it's a fun hobby and I've made lifetime friends as well.
The market for these is hot but I would generally not call it passive. Timing is critical on many cards and offloading them can be nontrivial. I do this lightly as well, mostly for fun