Do NOT leave that short answer blank!
Do you do anything else on weekends? Community service, church-related activities, etc. Do you help with the household that you and your father live in, in an unusual way? For example, do you do ALL the cleaning or cooking, manage bills or grocery shopping? Do you care for or tutor younger siblings? Do you have any individual activities such as writing, art, music, etc that you do, even if they aren't done via an organization.
You could talk about the bodybuilding in the EC short answer space, but you might want to write in some depth in the Addtional Information essay about why you don't participate in organized activities outside of school, and talk about what you DO do with your time when you aren't in school. And do you belong to any clubs or groups that meet during the school day??? I'm guessing you need to think a little bit more creatively about what you do with your time, and how to include that in your applications.