What do you do for fun?
This is a chance to highlight your relevant extracurricular involvement. Although firms are looking to hire those who are dedicated to their jobs, they also want people who are well-rounded and interesting. This is your chance to show the interviewer your personal side. However, remember to have relevant examples (i.e., "watching TV" won't win you many points).
Also, keep in mind that equal employment opportunity law mandates that any interview questions be job-related. Questions about what you do outside of the office sometimes skirt illegal territory, even if they are not as blatant as "Where do you go to church?" Your strategy should be to steer the discussion back towards work. For example, if asked what you do for fun, you might say, "My focus is work, but I enjoy playing tennis during my down time. It's an incredible workout and also a great way to clear my head and gain clarity for the rest of the week."
If asked a blatantly illegal question, you need to find a diplomatic way to turn your answer toward work yet not come across as a traffic cop. For example, if asked whether children are a burden on your work time, you might answer, "I don't have any family issues that would interfere with my ability to give this job my full commitment."