Though it can take a while to build up enough cash to put a 20% down payment on an investment property (the typical lender minimum), they can snowball fairly quickly. The key here is to correctly project income and expenses in order to calculate cash flow (the free cash you can put in your pocket after all associated property expenses have been paid). However you have to be sure to include the cost of a property manager in your calculations unless you want to manage the property yourself. Even with a property manager, you may be required to make large repair decisions every now and then – so while this is not a 100% passive activity, you are not directly trading your time for money like traditional employment.
Came to the U.S. as an immigrant in 1968 from a poor Asian country with only $100 in my pocket. Took advantange of 401-K savings plan by contributing 10% of my pay. My employer matched the first 6% savings (50 cents/dollar saved). Did not know anything about investment so 100% of 401-k money was invested in index 500. No other savings except 401-K. Retired in 1999 at 55 years old with about $1.2 million in 401-K and $450,000 lump sum pension which I rolled over to IRA. I invested this money in bonds and only buy equities (small cap index) whenever value drop to at least 50% of its high. I made a lot of money by investing in small cap index (ticker, IWM). Because of the risk involved, I don’t buy individual stock.
You've probably read blog posts and articles that recommend a certain brand of backpack or water, so you click on their hyperlinked link. Oftentimes, that person gets paid a commission when you do. If you have a blog, the same can happen for you. It's a win-win-win for everyone involved — you, the product you're recommending, and the person who clicks on the link to get the product. Pat Flynn talks about this at length on his website, Smart Passive Income, where you can learn a whole lot more on the topic, aside from affiliate marketing.

Venture Debt – I invested $120,000 in my business school friend’s venture debt fund. He started his own after spending 8 years at one of the largest venture debt funds as a Managing Partner. I’m very focused on income generating assets in this low interest rate environment. The true returns are yet to be seen, as the fund has a 5-7 year life before it returns all its capital.

These days, low-risk bond yields aren’t enough to meet most income needs. When investing in bonds, the income typically comes as coupon payments – contractually guaranteed interest payments at predictable intervals.  There are many kinds of bond income available, so you must strike a balance between reaching for higher income (yield) and limiting risk.