Hands down, something I did.
When I graduated high school, I promised myself that I'd live my life in a way that I'd have as few regrets as possible. A dozen years later, I regard that decision as one of the best I've ever made. It forced me to get out of my comfort zone, get my hands dirty and really strive to reach my goals. My regrets are positive ones, if that makes any sense. It's much easier for me to live with the thought of, "Even though I failed, at least I did my best" verses, "I wish I had done this...".
Has it been easy? No. Has it been worth it? Undoubtedly!