Owning real estate can be a great way to generate monthly income. You can purchase a single-family home, duplex, or even apartment building.
Most of us don't have the money to buy a literal building, but you can invest in real estate with an REIT instead. It's like an index fund for real estate and much more accessible for normal people
Although not as easy to start as some folks make it seem, blogging is a great way to earn income once you get established. Make money via display ads or affiliates.
If you prefer talking over writing, launch a podcast. The great thing about running a podcast is once you record, edit, and publish, you can wait as people download the episode.