The question about impressive EC’s comes up regularly on the forum. There is a thread with several posts by Northstarmom, a Ivy alum interviewer, about what constitutes impressive ECs from the point of view of the most selective colleges. The post is at http://talk.collegeconfidential.com/what-my-chances/210497-those-ecs-weak-so-what-s-good.html
As you will see from that link, at the most selective colleges they are looking for depth more than just participation. Stanford, for example, says