Asking those who have got in previously. 
Well, itβs a bit late now anyways,
Math Club/MathCounts/AMC8, Spelling Bee awards,
Instrument Lesson & Youth Orchestra,
Scouting & Community Service,
School service - teacher aide, tour guide, incoming student mentoring, Student council, etc
Sports (if any)
CTY talent search program honors
Local Gifted program Summer Science camps (selective)
Away from home camps:
3-week CTY summer camps
School organized 1-week tall boat environment science camp (selective)
ChoatieCandidate had:
Scouting (years of community service)
Trumpet/jazz band
Film/awards
No camps. It was a pretty short list, but scouting is a pretty deep EC.
Student govt
Martial art - 2nd degree black belt
School orchestra - cello
Varsity soccer but low competition
No service other than brief peer tutoring
I guess the community service plays a smaller role than it does for college application. Just a guess and can be wrong
Include anything and everything that you are passionate about and could spend a few minutes talking about in an interview. IMHO a shorter list of deep interests is better than a longer list of shallow ones.