Not really passive but almost. I sell bicycle bells on etsy[1]. It started with a friend that had the idea and later left the project. Initially I joined mostly because I was interested in finding a technical solution for how to print them.

The fun tech is that I use a pen plotter. [2]

I don't really make that much money selling them but at the same time I had a 400% increase in sales this year (Thank you etsy algorithm, and probably thank you pandemic). 0 investment in marketing, I just make sure I have stock (16h of work per year probably, but hard to measure). Also this year I moved to a place where the grocery store and the mail office are in the same place and 5 minutes from my home which is perfect.

I have a a bit too much fun optimizing processes. finding weird ways to automate the writing of shipping info in the packages.

The only thing I wish I had more time to, was at improving the drawings.

[1] https://www.etsy.com/shop/gunaPT

[2] https://www.evilmadscientist.com/2019/bike-bells-with-axidra...