Something you're just good at with minimal effort and/or you learned much more quickly than average.
For me, it's paper snowflakes. My brain just seems to effortlessly figure out what cuts to make to the paper wedge to make it turn out exactly how I want it. Largely useless, but good fun and was a much-needed ego boost when I was a kid :]
Running. I'm pretty fit and I work out (retty much only bouldering tho). Despite running on average once a month I can run a 45 min 10k. I think if I actually trained for running I could be pretty good but I just prefer climbing as a sport.
I've always had a natural grasp on running, but never really enjoyed running over 5k, and at anything above 10k I'm quite bad. If you ever decide to pick up running again you might find that shorter distances (1-5k) are a better fit for you, people are different :)