You should answer this question honestly. Remember that only 1 in every 2.1 applicants to medical school are accepted, so it is worthwhile for you to think about the answer to the question. I wish you the best, but it is useful for all of us to have a Plan B.
Last year's UMass secondary had a similar question: "If medicine were not an option what career path would you chose and why?" or something like that. I chose to talk about public health, since much of what I have focussed on recently revolves around public health. I think the best way to answer that question is to think about venues you have explored. What would you enjoy doing and why would you enjoy doing it.
I don't want to and have never thought of an alternate career. I have contemplated what it would be like to be a nurse, or a PA, but never considered actually doing it. I have wanted to be a physician since I was a kid and have had tons of obstacles..yet never wavered on that decision. Do they look at you in poor light if you tell them, "uhhh, never thought about NOT doing medicine??"
I won't be happy doing anything else, so why consider even doing something else?
Acupuncture! I've met many burnt-out MD's, massage therapists, nurses, physical therapists, chiropractors who have switched to acupuncture and are very happy b/c of the holistic approach, patient quality of care, and low malpractice.