Rentals, just like stocks, throw off cash. With rentals we call that cash “rent”, and with stocks we call it dividends. A significant difference however is that the S&P 500 has appreciated at ~6% per year (above inflation) for the last 100 years…..Real Estate has had almost 0 growth above inflation. So are rents higher than dividends? Maybe, maybe not. But unless you got one heck of a deal, the delta in rent over dividends will have a very tough time making up for the 6% per year difference in appreciation.
However, I think for those who are willing to do what it takes, the sky is the absolute limit. As an example, I’m trying to take a page out of FinancialSamauri’s book and create an online personal finance and investing blog. It is an enormous undertaking, and as a new blogger, there is a seemingly endless amount of work to be done. That said, I hope that one day I can not only generate some passive income from the hours of work I have put and will put into the project, but I hope to be able to help OTHERS reach their financial goals. 

I have already come up with 50 ways that a management company can screw you for profit without you ever knowing(or not finding out for awhile). Did you have an inspection before you made an offer on the property? Do you have a picture of the property you bought? How do you know if that picture shows the house you actually own? or if it even hows the ‘current’ state of the house you own?
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.
Investing in real estate: Investing in real estate offers more passive income cash potential - but more risk - than investing in stocks or bonds. You'll need substantial amounts of cash to invest in buying a home -- it usually takes 20% down to land a good home mortgage loan. But history shows that home prices usually rise over time, so buying home a for $200,000 and selling it for $250,000 over a five-year time period, for example, is a reasonable expectation when investing in real estate.
The terms on private money loans can also vary from very short (a couple of months or even weeks) to years. Once the money is loaned to an investor the passive income will come in as long as the investor can meet their obligations. The work with private money lending comes in the beginning when choosing a private money lender and if the loan ever goes into default.
For those who prefer a more do-it-yourself style but still want their investments to be managed automatically, a robo-advisor like Betterment may be better suited. After completing an initial questionnaire, this program will automatically invest your money based on things like your risk tolerance and time horizon. They’ll even rebalance your portfolio when necessary – all automatically, of course!

Creating passive income is the key to retiring, especially retiring early. If you want to retire early with the stock market you will have to save a huge amount of money, because that money has to last so long and retirement calculators depend on you using your investment to fund your life. With passive income you will have income coming in as long as you own the investment, without every eating away at the principle invested. Creating passive income may take more work than investing in the stock market, but I think it is well worth the extra effort!