The more I deal with ungrateful patients and have to be away from my family due to work, the more I become a huge fan of passive income. Every 6 months when I get a check for my UpToDate sections I worked on 4-5 years ago that only require periodic minor updates, I’m always reminded how nice passive income is. Rental properties are great too, but I completely agree that you must do your homework. There are a lot of bad rental properties that will not only fail to provide passive income, but can cost a great deal out of your own pocket.
I have only dabbled in drop-shipping before when I had an eCommerce platform 6 years ago or so. I think it is something that you could do on the side, but you would want to do in depth research on the industry you want to get into before setting up shop. It may be a little less passive up front, but over time you could take your hands off the wheel.