I'm not sure I'd want to get into the rental business given the vast pool of less than desirable renters, but I have a friend who does pretty well with it.
I have rent houses. Had those when I was working in the oil and gas business. Retired from oil and gas and acquired real estate license. I personally think buying rental property, or buying a home and selling it and being the mortgage company.... this is the best way to invest during retirement. Provides diversification from the obammy market, and you own real assets, or have a real asset behind your investment.
I do work a lot with investors who are buying rental property.