First of all, let's face the facts. Working 9 to 6, you won't have any time during the day. If you choose to have a second job during the evening hours, you'd get burnt out very easily. Chances are that you might under-perform in both the jobs. I recommend having a side business rather than a second job. Some investment of money and time would be required at first. I have come up with a few options that would require less work from you but would be a decent second source of income.

Dividend Income: Dividend income is wonderful because it is completely passive and is taxed at only 15% if you are in the 25%, 28%, 33%, and 35% income tax bracket. If you are in the 39.6% income tax bracket you will pay a 20% tax on your dividends. My dividend income portfolio mainly consist of dividend equity and bond ETFs such as DVY, VYM, MUB, TLT, and IEF. Total stock and bond income is a little over $100,000 a year due to a heavy accumulation of stocks and municipal bonds after selling my house.

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For a person working for a wage, their source of income is their job, or their labor (aka their time that they sell to someone). But you can own a house, and rent it out, and when your renters pay you renter every month, the source of that income check is your rental property, or your house. It would just be called rental income. If you have money in the bank in an account that pays interest, those interest payments are another source of income. We can say the source is interest payments, or we can identify the asset that is generating rent, and say your money in the bank that is generating “rent” (aka interest) is your source of income.