Tennis
I first played tennis as a child, two years of training and then a decade of nothing. I picked it up again abruptly and with more commitment than is probably sensible.
I had always wanted to get into fitness. For a few years I tried the gym and yoga, and neither of them clicked for me. Then it occurred to me that I had played tennis as a kid and should get back to it. I knew it was the right decision on my first day back on the court, when the feeling was "Oh, how much I have missed you."
The academy is near my house, and every session there is full of fun and camaraderie. As a kid I played a single-handed backhand. Now I play a double-handed one, since I see many more benefits in it, and my game is an all-court game.
A player's own head is a harder opponent than the one across the net, and keeping it quiet takes a lot of practice.