What's the most underrated way of earning "passive/side" income?

Wherever I go, I see such cliché advice - become a content creator, design a course, conduct webinars etc.

When I was a kid, I remember seeing my neighbour being a freelance carpenter despite having a full time job. Apparently it makes a lot of money, even now I think.

What are some other ways, truly underrated to get em cash?