There is no one summer activity that is the best activity for college admissions. In general terms, the best activity is one that enhances your application to college. What does that mean in practical terms?
Colleges like to see students with a passion. If your passion is playing soccer then you might want to volunteer helping coach a junior soccer team. Or if you need to earn money, then find a job coaching soccer or working at a sport store as the soccer “expert”. If you think you might be good enough to be recruited to play in college, then you might consider attending a soccer camp or playing on a traveling team.
Doing something related to your passion is more important than what that passion happens to be. Colleges are looking not only for athletes but musicians, student leaders, researchers, dancers and many other types of students.
Don’t worry if you have to work to pay for college or even help support your family. Colleges consider work just as valuable an activity as any other activity. But if you have to work, try to find an activity that also is related to a passion of yours.
Some people think that to be a good summer activity it has to involve foreign travel to help third world orphans or some other worldly good. This is not true and in fact these type of expensive summer activities can sometimes backfire and make a student a less attractive candidate. Simply having the money to spend the summer with foreign travel does not impress admissions officers. Most admissions officers do not make much money and they may be offended if you come across as the rich kid who tries to impress colleges with their money. If you are interested in some type of international work, then international travel can be helpful. Again, the issue is your passion, not your money.
If you spend the summer developing your passion, whatever it might be, you will be doing what you like and enhancing your college application at the same time. Now go out there and figure out what you can do to enhance your passion.