There are dozens of ways to generate passive income. However, the option you select has to do with two metrics: time and money. Either you have a lot of time or a lot of money. Most people usually don't have both. But, if you have a lot of money, generating passive income almost instantly is easy. You can buy up some real estate and begin enjoying rental income. Or, you can invest in a dividend fund or some other investment vehicle that will begin generating a steady income for you.
Nobody gets early FI investing in bonds, CD’s, or even stocks unless they make a huge income or are extremely frugal or a combination of both. Paper assets just don’t provide enough returns. Business income can be great but it is typically not as semi-passive as I would like and there is a relatively high failure rate. That is if you can monetize an ideal to begin with. RE investing needs to be higher ranked IMO as a way that the “average guy” can become FI.
If you like to be involved in the financial markets but have no interest or time to pick or analyse stocks, a good proxy can be dividend investment funds in the form of exchange traded funds or unit trusts. This put your funds in the hands of fund managers to pick out dividend stocks with dividends being paid out to unitholders. Similarly, there are exchange traded funds invested in basket of income instruments such as REITs with the dividends being paid out. For such diversified instruments, your risks is broadly market risks and volatility resulting from market fluctuations and industry wide risks.

It’s a (mostly) short term, higher risk, higher reward place to invest cash that has a low correlation with the stock market, but is far more passive than buying and managing properties, has more opportunity for diversification than private placements (minimums of 5-10K, rather than 100K), and most of the equity offerings (and all of the debt offerings) provide monthly or quarterly incomes. Unlike a REIT, you can choose exactly which projects you wish to invest in.
In equity valuation, residual income represents an economic earnings stream and valuation method for estimating the intrinsic value of a company's common stock. The residual income valuation model values a company as the sum of book value and the present value of expected future residual income. Residual income attempts to measure economic profit, which is the profit remaining after the deduction of opportunity costs for all sources of capital.