Cheeky-and-also-serious response:

Soil, for the food you grow to eat, share, or sell. Much about a green, growing area is passive income for is.

Another option is lobbying your elected officials to regulate in the interest of healthier land, water, and air, which will help you and many others.

Making something useful and giving it away (or charging a nominal amount for hosting costs, or asking others to help with that to keep the use-barriers low) is an investment in other people that may continue paying dividends long after you’re dead.