Let’s use some critical thinking skills. Why is Harvard asking this question?
IMHO, it’s because Admissions wants to know MORE about who YOU are, how YOU think, and what intellectual activities YOU’VE pursued outside the strictures of the classroom. Admissions wants to understand YOUR commitment, drive, dedication and determination in self-directed learning. They want to know why YOU do the things that make YOU uniquely you.
While I’m sure some students will submit a laundry list of intellectual things they’ve accomplished outside of school – that’s boring, as it lacks an explanation for “why you do the things you do” and that’s really what Admissions wants to know. So, I think it’s best if you expounded on your intellectual interests outside of the classroom and why they are important to understanding who you are as a person.
If you haven’t done much intellectual activities outside of the classroom – that’s okay. I’m sure many athletes (recruited and non-recruited), musicians,and artists haven’t had much time to pursue intellectual activities outside of the classroom as most their time has been in practice, athletic contests, rehearsal rooms, performance, and the art studio.