Hey there, theres quite a few things you can do to strengthen your application, if you can, take the SAT as this can be really I think registration has closed for October but November should still be open. In terms of extra-curriculars try take an EPQ if your school offers one, Ivy league universities like students who do research in high school. If your school participates defo make sure to take part in the Senior UK Maths challenge and try to get silver or gold, theres also the Bebras Computing challenge coming up in soon so do try to do well in that.
Outside of those the Rise for the world challenge is a great idea as this can help you to start up a passion project which is great for the Ivies, deadline is in January though so do make sure to get started ASAP. If you're applying next year you could start looking into summer programs and internships, particularly selecitive ones for Maths and Computer Science, Stanford, MIT and Harvard offer several of these. If you havent already its good to find a spike or niche in computer science, e.g. Quantum computing, ML, Cloud Computing etc so you can really specialise in one of these potentially making apps or basing some larger project around one of these. I'll have a look for links to a few of these things and send them here when I do.