Nothing. As in there's nothing simple that will significantly improve your existence.
Questions like this always bother me because it's like saying you want to make your life better with minimal effort. But the truth is the amount of positive change you can create in your life is almost always proportional to the amount of effort you're willing to put in for it. You want something simple? Fine. Odds are it'll have a negligible long-term impact. You want something better? Then you have to make real, serious changes that involve hard-work, introspection, and no small amount of pain and suffering.