If you design a wildly popular app (which is harder than it sounds), you might be pleasantly surprised with the income they generate. Offering one or more apps at the iTunes or Android app store gives your creations wide exposure to prospects. And income can be generated by charging for the app, displaying in-app ads, or charging for in-app features and upgrades.
Drive for Uber or Lyft: If you're in a locale where you can find Uber or Lyft (or even one of the many competitors around the world such as China's Didi), you could easily make a respectable income. The hours are flexible and you can work as you see fit, making it perfect even if you currently have full-time employment but are looking to make some money on the side.