The wrong answer to this question: it depends who's asking
The right answer to this question: an honest answer. As a former MBB interviewer, if I'd ask this question, it would be so that I can see what you care about other than consulting, work, or B-School. If you have a specific hobby that you do during vacation, say so. If you can demonstrate that you know when you need to do nothing except sit on a beach, do so.
Bottom line, just be authentic and show some parts of your personality.
Bonus marks - determine if the interviewer is interested or shares common interests. If so or if not, turn it into a conversation, not just a story.