Sit back and let the money come to you.
I had a conversation with a friend once about what kind of job he’d want if he could have any job in the world. He said he wanted to own a parking garage. When I asked why, he said “it’s the ultimate passive income. People give you money to park and it doesn’t require a lot of upkeep.” It’s a nice dream, but we can’t all own parking garages. Still, while not quite on that scale, there are practical ways to create a passive income stream for yourself and score a few extra dollars.
Being a full-on entrepreneur is obviously a big commitment of time and energy, but if you’ve got an idea and the means to put it into action, you can start it off on a small scale so it doesn’t completely take over your life. If you’ve got any coding or programming chops, you could try your hand at developing a mobile app or game. Remember Flappy Bird? Simplest game in the world, but it made so much money that the guy who made it started feeling bad about it. If computer stuff isn’t your jam, you could make physical goods like T-shirts or simple knit crafts. You can sell handcrafted stuff on an Etsy store, or clothing designs on Redbubble.
If you’re particularly well-educated in a subject, pass the knowledge on. Try creating an online teaching course, or write an educational book. There’s always a market for picking up skills, and if you’ve got skills to offer, people will pay you to teach them, one way or another. If you happen to have some spare real estate somewhere, whether that be a house or apartment you’re not using or a room in one of those, you can rent it out.
Unless you get exceptionally lucky, you probably won’t be able to replace your regular job with any of this stuff, but if you’ve got the spare time and some kind of marketable skill, then what’s the harm in setting something up? Make some extra cash, and go on with your life.