Rental properties are defined as passive income with a couple of exceptions. If you’re a real estate professional, any rental income you’re making counts as active income. If you’re "self-renting," meaning that you own a space and are renting it out to a corporation or partnership where you conduct business, that does not constitute passive income unless that lease had been signed before 1988, in which case you’ve been grandfathered into having that income being defined as passive. According to the IRS, "it does not matter whether or not the use is under a lease, a service contract, or some other arrangement."
If you have specialized knowledge in a certain topic, you can put together an online course to teach others. For example, if you have experience in real estate investing, you can create an online course “Real Estate Investing 101”. The benefit of an online course is that once you create the course material, you can sell it to as many people as you want.