In addition to the investment income strategies that you often hear about like bond ladders, ETFs and dividend stocks, this former college professor also includes covered calls in his list of five retirement income strategies
. He goes into to some details of covered calls in this post, "The Quest for Yield (Part 4), Covered Writes, The Single Best Method for Generating Income
I'm sure many would consider covered writes as not worth the risk and the effort for retirement investors, but I thought it was worth to at least mention it. You'll have to do your own research to understand the risks.