If it's to be passive then some form of physical asset that you can rent or lease would seem to be necessary.
A friend of mine decided to invest in the machines that go in public laundrettes. Not sexy, as even he freely admits. But they do deliver steady, dependable cash flow against an asset that is generally reliable.
If you can raise finance at cheaper rates than you can rent out, there are so many possibilities. A fleet of bicycles in a tourist town maybe? Sub letting a basement?
The only thing about passive income is that it often requires an initial cash outlay. And someone to manage the assets.