Most colleges actually do care mostly about grades, but the most selective ones, including Stanford, have so many applicants from the top end of the GPA range in hard classes that other things like extracurriculars, essays, etc. become much more important in differentiating admits from the pool of applicants with top-end GPA in hard classes.
As far as extracurriculars that matter, in order:
- Be a good enough athlete in a sport that colleges find you to be an attractive recruited athlete.
- Do something that shows the highest level of achievement or recognition (international > national > state > local > school).