Besides your GPA and test scores, colleges are interested in who you are beyond your numbers as well. Extracurricular activities (ECs) tell you more about as a person to college admission officers. You may have world-class skills and qualities that are not reflected on your academic performance.
The media has told us that a well-rounded student will have a much better chance getting applied to "good" colleges. This student will have top grades when volunteering hundreds of hours in multiple groups, clubs, and organizations. However, colleges prefer focused and meaningful ECs of a spike student.
As a matter of fact, colleges have a cap on how many ECs you can list on your application and how many hours a week, how many weeks a year you spend on it. If you're not careful with what you put in, you can accidentally put 80 hours of doing ECs each week. For reference, a week has 168 hours and you probably need almost 100 hours to sleep and study at school. Also, please don't lie on your college apps. The Earth is round and you never know who could be reading your college application.
A spike student has decent grades and test scores and just a limited but extremely meaningful ECs. College admission officers read hundreds of college apps, they want to look for interesting ECs. Don't get me wrong, you don't need to build a shelter for children in Africa (though it would be pretty impressive and equally costly). Still, colleges want to see your passion and to prove this, you need to show dedication.
The best way to show dedication is to show your progression with your activities throughout a long period of time. For me it was powerlifting. I was super overweight after my freshman year. At the end of my sophomore year, I decided to go the gym and 2 years later, I was competing in powerlifting while being much happier with my body image. It's okay if you don't achieve something great with your activity. What really matters is your effort.
If you are really passionate about your activity, you should be able to write a formal essay about it without even trying so hard. Reflections are just as important, you can make a seemingly normal EC become extremely impressive if you can tell the admission officers what it means to you.
You also want your ECs to tell your college your interest in your major if possible. For pre-med students, the ECs would be working with patients and shadowing physicians, for example.
If you just volunteer for the sake of getting the activity on the college apps, you're wasting your time. Again, these people are paid to read your applications and they are expert at it. They can tell when you're just another typical high school student volunteer an hour a month at a local hospital or become a president of a club which holds random meetings.
Anything outside of your school time is considered ECs. Yes, taking care of your siblings is considered an EC. Here is the list of some possible ECs you might want to start over this summer if you haven't found something you're passionate about.