Answer: Exercise.
Explanation: Quoted from "Is Happiness the Beginning or the End?" By Jamie Anderson. "The good news about Achor's research is that it doesn't just apply to the optimists among us. We can all train ourselves to be happier, by following a five-part process of gratitude, journaling, exercise, meditation, and random acts of kindness. He claims that real world experience proves that just three weeks of practice yields lasting results."