I did sci oly, and it is a club that takes up a lot of time. Do you have any awards/medals? Try finding places to volunteer over the summer that are meaningful to you. I’m really into robotics, and I was able to teach other kids over the summer what I knew, and I got a lot of community service hours from it.
Edit: For something CS related, you could develop an app and put it on the App Store or the Play Store, as a good side project. I don’t think you need to “demonstrate interest” for CS.