As @MWolf has suggested, the Ivy League and top 20 universities are not all the same. If you want a good chance at getting into any of them, then you should spend some time figuring out what you want in a university, and you should also spend some time finding out what the differences are between the top schools.
One good place to start is to do a few university visits. You should be able to get a guided tour, and also at many schools you may be able to sit in on classes and talk to professors.
One thing to think about is whether you would prefer a large school or a small school. There are some pluses and minuses each way. There are also many very good small schools (which in the US are frequently called Liberal Arts Colleges). I have a daughter currently studying biology at a small university and she is doing very well with relatively small classes taught by full professors (which gives an opportunity to get to know the professor) and she has found a great research opportunity over the summer. Large universities will have a wider range of majors and will have some resources that the smaller school don’t.
Also, do not stress yourself out over trying to do whatever a top 20 university might or might not want you to do. There are a great many very good universities. Many students do very well with degrees from their in-state public university or from very good small schools that many of us have not heard of. If you look at the graduate programs in top 20 schools, a lot of the students there will have done undergrad at their in-state public school.
In terms of ECs, I have consistently felt that students should do what they find interesting. If you participate in ECs that you like, you are more likely to do well. I literally spent no time whatsoever in high school thinking about what ECs a university would have wanted me to do. I just did what I wanted. I still got accepted into a “top 3” university, and doing well in my ECs probably helped.