I personally haven’t tried any of the below methods so I can't speak from experience to how they work. In my mind, the most lucrative way is through building a REAL business online and providing your visitors with great value. However, after some thought, I decided to include these here because, quite often, the biggest roadblock for many is a mental one: believing that making money online is possible. From my experience, once you have made your first dollar, or even your first cent online, the world is suddenly a different color.
My favorite type of semi-passive income was rental property because it was a tangible asset that provided reliable income. As I grew older, my interest in rental property waned because I no longer had the patience and time to deal with maintenance issues and tenants. Online real estate became more attractive, along with tax-free municipal-bond income once rates started to rise.
Though it requires a huge up-front investment (well, at least 20%), real estate is awesome, because it’s (usually) pretty stable, and it’s a triple-whammy: you make money by leveraging a bank’s money, you can rent it out with almost no work at all day-to-day, and the value of the underlying asset INCREASES. This is why so many people flock to AirBNB to rent out their apartments when they’re going on vacation, too. It’s pretty passive, you don’t have to own the asset, and you just hand over keys and hire a cleaner before/after. The value of your apartment is untouched. By contrast, imagine buying a plane and renting it out to people for tours — you pay tons of money to maintain it, and after 20 years, it’s value is almost nothing. The exception is if you can manage depreciation and somewhat automate the value-delivery. If you have an expensive tool (like a 3D printer) and charge people to come over to your house and print things on your 3D printer, it’s pretty passive for you to answer emails and open the door, so I give it a “Semi to yes” for passivity.
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.