Use something that makes you unique. Colleges are always looking to diversify their field of students. Put any awards or talents that do not make you like the average student. Another way to look at is is like a job interview. If an employer asked this question, how would you answer.
You could write your favorite subjects in school. You could also try to say what you did for extra curricular activities. Tell your hobbies, anything you would share to a new friend, but in a professional type of way. I hope this helps and good luck! :D
