Investing in bonds: Similarly, bonds are an attractive way to engage in passive income. Over a recent 45-year period, bonds funds, as measured by Vanguard Funds, returned 7.1%. Of course, there's no guarantee that investments in stocks or bonds will always work out well, investing in them is by far the surest way to generate money through passive income.
If you're looking to get started in real estate, look at a crowd funding solution like RealtyMogul. It works similar to LendingClub - you commit as little as $5,000 towards a property. When the property is fully funded, you become an owner, and will receive your share of the earnings and appreciation in the property.  Check out RealtyMogul to learn more. 

The topic of buying and selling blogs deserves it’s own book though and is a very difficult skill to master I must say.  I rant about the greatness and simultaneous pitfalls of marketplaces like Flippa elsewhere on this site, but needless to say I think this is it’s own expertise; worth learning more about for a lot of us who know that starting a business (getting momentum) is a lot tougher than fixing up and steering an existing business to greater profits (maintaining and directing momentum).
Also, I know from my personal experience that when you only do something because you think it’s a money-spinner you end up nearly killing yourself to get the idea over the line.  I remember creating a careers advice site that bored me to tears.  Every word I wrote, hurt a little.  All 200,000+ words!  Do not put yourself through this pain.  I know I won’t ever again.