The sky is the limit but some alternative income ideas are better than others.  Stanley and Danko in The Millionaire Next Door, are quick to point out that millionaires allocate their time, energy, and money efficiently, in ways conducive to building wealth.  That means that a millionaire is much more likely to spend Saturday mornings checking on his rental property rather than cruising garage sales looking to add to his garden gnome collection.  Robert Kiyosaki of Rich Dad Poor Dad fame likes to emphasize the benefits of a passive income.  The books aren’t exactly a prescription for wealth, but he makes two excellent points.  The first is that a stream of income that provides money when you’re sleeping is preferable to one that requires you to work.  The second is that passive income often enjoys much better tax treatment than the income from your professional work.  Here are some income streams that are fairly common among physicians:
Two months into my work hiatus and I’m doing well. I’ve made a little bit more than I’ve spent and I’ve worked about four hours a day on average. Ultimately, I’ll know I’m ready to make the permanent jump to a 4 day work hour when my passive income plus side incomes equal my day job income. Until then, I’m going to go back to my day job and keep grinding it out.

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.