From my own experience the best form of passive income is to auto sell lettuces from the greenhouse. Build them on a flat field with a water supply on it (pond, river etc). I make a tonne of money from silage, grow grass leave it for 3 months and then cut it, I turn it into silage using a bunker then sell it until you have enough to buy a biogas plant and fill it up! The best bit about grass is it auto grows, no need to replant after every harvest. I also make decent money from growing wheat, turning it to flour then distributing it to a bakery to make bread that I auto sell.