I have two such side projects that run almost passively, neither is an app though.

1- An extremely simple habit tracker spreadsheet template that I also use myself - https://www.preetamnath.com/habit-tracker-template

Makes ~$100/mo but this month it's on track to make $200, probably due to new year's enthusiasm?

2- Learn Programmatic SEO, a straightforward course to help people identify repeatable keyword patterns and use that to drive traffic to their business, website, projects etc. SEO beginner-friendly - https://www.preetamnath.com/programmatic-seo

The course makes between $300-$500/mo.

I don't have any costs other than website hosting and Gumroad fees. I do think these projects have scope to improve and grow. Would love feedback from anyone who finds them interesting!