Assuming that the risk is managed and your own involvement is limited to only the spare time you can't productively employ otherwise, there are a lot of passive income avenues. But don't forget, to earn passive income, you'll have to invest either time/effort or money in consideration. Or else, they have unavoidable downsides. Here are a few examples.

1-Property Rental (Financial investment needed)
2-Youtube channel monetization (Deep expertise and fanbase needed)
3- Vacation getaway management(Financial investment and self involvement required). It's a full time job but doesn't feel that way.
4- Royalty (Write a book or sell a self developed mobile application)
5- Share dividends (this is as close to a 100% passive income as it gets. RISKY)
6- Bank deposit interest (NOT lucrative)