If you don’t want to write the book yourself, you can also hire a ghostwriter through various online sites like Upwork, Freelancer.com etc. writing is the best passive income ideas for 2018. See my Kindle Direct Publishing Portfolio HERE. Selling your eBook can be a great way to earn passive income forever once it’s released. You need to take significant time to make sure it is high-quality. Also, you will need to spend time promoting your eBook.
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.