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 :]
I'm pretty good at sensing the emotions of people around me. It's not magic like some people think, but an obsessive awareness of small facial and body movements.
Oh, and writing dialogue is super easy for me, not sure why some people have a hard time with it.
These are possibly related. I am absolutely terrible at sensing people's emotions around me and understanding body language, and not so coincidentally, I am absolutely terrible at writing dialogue.