Well, a while back, in my ridiculously busy, sleep-deprived state, I was set on the most boring (but practical) answer to this question.
The ability to live without needing to sleep.
Don’t get me wrong, I always hit the snooze button on a Sunday morning, and I gratefully flop into bed at the end of a long day. But sometimes, when I’ve just got so much stuff to do, stuff that I need or even want to do, I wish I could skip the nightly shutdown.
Anyway, let’s get to the prompt.
Write about what your character would do if she never had to sleep. Read more books? Exercise? Plan vacations? Become a dashing, leather-clad vigilante?