What’s a ‘lazy’ side hustle something passive or semi-passive that engineers could try?"

Looking for low-effort side gigs for engineers

I’m a designer, but I’ve always been curious about the technical side of things. I see a lot of engineers with deep skills, and it seems like there must be some clever, low-maintenance ways to put that knowledge to work for some extra income. What are some genuinely passive or semi-passive ideas you’ve seen or tried? I’m thinking of things that, once set up, don’t require constant daily attention maybe something with digital products, automation, or leveraging existing assets. What’s worked in your experience?