Everything and nothing.
There is no secret formula. If you composed a concerto which had its debut at Carnegie Hall by the NY Philharmonic, or won a Pulitzer prize, that would do it. For mere mortals? There’s no checklist which says “Intel semi-finalist adds another point to your GPA”. There are kids who get into Princeton who collect butterflies and do crafts; there are kids who get into Harvard with world class accomplishments.
Work hard and read a lot; foster your own interests and curiosity about the world; participate in a few things that are meaningful to you. After that- it’s a $%^& shoot for everyone, as you well know. Unless you’ve already composed that concerto- in which case, just send in your application and wait for the results to come in.
There are dozens of colleges which would be happy to have you just the way you are so don’t pursue EC’s you aren’t interested in for the sake of college admissions. Because if it doesn’t work- you’re stuck fencing when you’d rather have volunteered on a political campaign.