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 :]
Good on you for just casually getting a computational physics degree without inherent math talent... like holy shit that's impressive!
I have also cried over coursework on linear algebra as well as electricity and magnetism :') Brutal stuff.