Investing in coins and collectibles: Buffalo nickels and Spiderman comic books are good examples of coins and collectibles that can rise in value, and thus offer opportunity for passive income investors. You'll need to get up to speed on the value of any coin or collectible under consideration, but once you do so, you're on the way to price appreciation on a commodity you'll be paying a lower price to buy, and will garner a higher price when you sell.
As I’ve thought more deeply about how to answer this question over the years, I’ve come to a realization that the problem is not the answer, but the question itself. For those who’ve asked it, I don’t think it’s always coming from a place of “quick money.” If we reframe the question, I think there’s room to empower and actually help those who’ve asked it—to give them a foundational understanding of what it really means to generate an income online.
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.