One of the best ways to make money online that will continue as passive income overtime is by creating a blog or a website on a high paying niche that you have a lot of knowledge about.
In the beginning you won't earn much if any money at all but as the years go on and you continue to work on your blog or website and add content then the money will continue to come in year after year even without doing much work.
But in the beginning you have to work hard on your blog or website to get the website traffic and add content.
After working around 5 years on your website then you can slow down on the work and the money will continue to come in as you sleep.
You can earn this passive income on your blog or website by monetizing your blog or website with Adsense or affiliate programs.
Or even sell your own products or E Book.