Other than athletics, what did you do during the summer and school year?
Don’t look for a prescribed path from other people 
In general, doing things outside of school can be beneficial because it requires initiative and can broaden and deepen your interests.
Most admits to Harvard, especially unhooked have unique extracurricular that will make them easily identifiable. This a college confidential, not likely to happen IMO.
I strongly disagree with that statement. Granted this is anecdotal evidence, but many unhooked students I know had very ordinary EC’s. For example, my daughter was an unhooked admit and her EC’s were theater and cheerleading. Her freshman roommates EC’s (also unhooked admits) were art and music. What WAS extraordinary was the commitment each of them made to their EC’s, which encompassed years of time after school, on the weekends, and during summer breaks.
FWIW: This article will help you understand what colleges like Harvard are looking for in an EC – and it’s NOT uniqueness: https://www.huffingtonpost.com/marjorie-hansen-shaevitz/extra-curricular-activities-college-admission_b_3040217.html
^@gibby I got your point. The OP’s question is if EC is the differentiator in application (make your application stand out) what are they? If the answer is just long commitment without distinction then I imagine many applicants will possess that. If it involves regional/national level recognition then I think it makes the student potentially identifiable.
An activity can be done with distinction, but not with regional or national level recognition, and be helpful. Sometimes letters of recommendation from someone who knows the applicant well in the extracurricular context can help. Too many miss this point. And an arts/music supplement can help too (recording or portfolio, resume, letters of recommendation). So there are ways to let schools know a little more about the depth of commitment to an activity, beyond listing it on a line in the Common App.