Diving.
Although I learned how to swim in elementary school, I never felt very comfortable in water. This was probably due to high school, where we had to swim laps and then play water polo, where getting forced underwater abruptly was a common occurrence. Many, many years later, I took a vacation to Thailand and asked a coworker where I should visit. He said to go to Koh Tao and while I was there, I should take the diving course. I did. Over four days I worried I would miss something in the lessons, setup my gear wrong and accidentally drown myself. Of course, the instructor was very on-the-ball and made sure we were safe. The course tired me out every day, probably from absorbing the new information and perhaps from anxiety. Once the lessons were over, I did some fun dives. It was much more relaxed and I really enjoyed myself. I've been taking diving vacations every year since.