“Start-up time” required: Poor – Experienced niche site builders will be well versed with outsourcing certain tasks and running through the whole process very efficiently, but when you first start out, it can take a long time to earn your first dollar from a niche site.  Doing research, designing the site, writing content, and doing proper SEO work can all be very time consuming, and the payoff isn’t always something you can rely on.
What's crazy is that my book income is more than my SF condo-rental income. Yet I didn't have to come up with $1.2 million of capital (the minimum cost to buy my condo today) to create my book. All I needed to create my book was energy, effort, and creativity. I truly believe that developing your own online product is one of the best ways to make money.

CBDT Chairperson, Central Board of Direct Taxes Investigation Division of the Central Board of Direct Taxes Central Board of Excise and Customs Chairperson Central Board of Excise & Customs Income Tax Department Central Economic Intelligence Bureau Directorate of Revenue Intelligence Tax Administration Reform Commission Goods & Services Tax Council
Investing in coins and collectibles: Buffalo nickels and Spiderman comic books are good examples of coins and collectibles that can rise in value, and thus offer opportunity for passive income investors. You'll need to get up to speed on the value of any coin or collectible under consideration, but once you do so, you're on the way to price appreciation on a commodity you'll be paying a lower price to buy, and will garner a higher price when you sell.
There are a ton of ways to diversify your investments, some of which can send real income your way. By opening a brokerage account and investing in ETFs or mutual funds, you can earn real returns you can use to supplement your income. Of course, the flip side can also happen – as in, you can lose money. So, make sure you understand the risks before you dive in.