The interviewer wants to get a glimpse into your personality and what types of things you enjoy. They are also looking at how you manage stress, since most activities you do in your free time are stress-relievers. When answering, make sure the things you like to do will not be construed as unethical or immoral, such as going to bars or nightclubs. Not everyone will think those types of events are unethical, but there are some interviewers who might think that way.