This does not mean you should go join every sports team in your school along with the school play. Most colleges prefer a clear interst in one area, especially one which shows leadership abilities such as organising AIDS walks once a month, or starting a school drama club.
An interesting way to get new ECs is to do something interesting with the summer prior to applying to college. I personally travelled to South Africa on a conservation trip. I made some fantastic friends, saw things nobody had ever seen before (seriously) and even though it was so fun, it is highly regarded in a college application.
Alternatively, you may want to join your school government and try rise to the top or maybe if you're looking to teach help out in lower years at school. Your ECs should reflect your major though; so if you want to go into politics join the government, for film, the drama group is good etc.
Organisations...
African Conservation Experience
Cross Cultural Solutions