What are some good EC's for Comp Science that will impress Ivies and Stanford?

I go to a very large high school with 6000 kids and over 200 clubs so this might differ for you, but join a robotics club, a computer science club, or even make your own club that you suits your interests. Look online for organizations and internships, or even volunteer opportunities that deal with computer science. Don't be involved in many clubs but show dedication to a few and even have leadership roles if you can in these clubs. That's what they look for involvement and dedication. Don't involve yourself in too many clubs and not dedicated to any of them, select a few clubs, internships and dedicate yourself to them. Hopes that help.

Colleges look for two things in EC profiles: breadth and depth. You'll have ten 'slots' so to speak on your application where you can enter in anything from clubs to volunteer work to jobs and summer programs—while ten sounds like a lot, those all add up quickly. They'll want to see that you tried different things and explored your interests fully, so trying a variety of different activities is usually a good call. They'll also want to see you go really deeply into a few of those activities and gain significant responsibilities and leadership; especially for Ivies and Stanford, having leadership experience in multiple clubs will be much more important than the specific types of clubs that you join. They want to see you take initiative and use it well to show your potential for a college environment.

Like what @saige and @jcdenton mentioned, if it's possible for you, gaining leadership positions in clubs will look better on your application than just joining a club and participating casually. Since you're a freshman it's a great time to get involved and if you stay with it throughout highschool you should be able to reach a leadership position.

Consistent volunteer work will also look good. This doesn't have to be CS related but if you can find something that is connected to CS it would be a good idea to pursue it. Depending on your current CS knowledge maybe you could teach coding to elementary school kids? Or do a computer literacy class for older people. If you have the opportunity, an internship or shadowing someone in the CS field can look good. Or even a job which gets you around tech.

I also found these two articles from CollegeVine that could help you. https://blog.collegevine.com/extracurriculars-to-consider-if-you-plan-to-study-computer-science/ and https://blog.collegevine.com/extracurriculars-for-aspiring-computer-science-majors/.

Bottom line is you're going to want varied activities on your application and you'll want to be deeply involved with a few of them. The ones you are deeply involved with don't have to be CS related but if that's what you're interested in then try to have at least one of your ECs that you stay involved with long-term CS related.