I always advise clients to develop both a breadth and a depth of experiences.
Admissions officers are looking for what I call a “T” shaped applicant. This means they prefer applicants who have a breadth of experience (such as participating in clubs, sports, music, etc.) and a depth of experience in one or two areas that they are particularly passionate about. Admissions officers expect that you will try a number of activities over your high school career; you are meant to experiment with different interests at that point in your life. But admissions officers also expect that you will have one or two activities that you love and are passionate about. They want to see you develop yourself in these areas as much as possible. Exactly how do that, however, will depend on your interests.
For example, if you are really interested in robotics, don’t just build them on your free time, but join a robotics club, participate in competitions, create a website to show off your work, create educational videos to share your robotics skills with others, etc. Also (if possible) take high school classes that align with this passion for robotics, such as physics and programming, so that you can show admissions officers just how much you LOVE robots. This is just an example, but you get the idea.
The key is to find something you are honestly passionate about and develop your skills and experiences in this area. You do NOT want to do this with a bunch of activities, and you do NOT want to do this with something you are not actually passionate about. Admissions officers review applications for a living, and they know how to spot applicants who are only involved in extracurricular activities to improve their university applications.
Admissions officers also like applicants who show leadership skills. If we continue with the robotics example, you could add leadership skills to your application by becoming the president of your robotics club, or you could start a robotics club on your own - whether that be at your school or in your community. You could also turn your love of robotics into a volunteer experience, such as teaching robotics to underserved (poor) children.
In sum - you want to use your extracurricular activities as a platform to show off who you are, what you are interested in, and what you can offer a campus above-and-beyond your grades. It doesn't matter what you do, so long as you sincerely enjoy it and develop yourself in this area/these areas as much as possible.