On the ECs question, I don’t know which are dazzling, but some to consider for a HS student who can already do some programming are FIRST (as mentioned earlier), USACO, topcoder (adults compete too), hacking CTFs (carries weight with CMU), maybe ACSL (maybe not rigorous enough), or even just learning several languages self-taught. Before USACO, my son did Project Euler. For someone interested in game programming, Ludum Dare is fun and a challenge. A bunch of kids here on CC mention App Store apps they have created and sold. Teaching programming to younger or underprivileged kids can be a good EC. If you have some programs you can show in the maker/slideroom portfolios that some colleges allow in applications, that can be useful.
My son has a friend who as a rising junior went to interview for a summer internship at a small but relatively well-known software company in our town. The programmer interviewing him wanted to give him an idea of what programmers did. He said, “Here, let me show you some of our actual code. I’m trying to find a bug that causes ____” and scrolled fairly rapidly through the code. Son’s friend said, “Well, there’s your problem” and pointed out the bug, which the programmer hadn’t actually found yet. He’s been a part-time employee ever since–works Friday evening and weekends. I don’t know if that is a dazzling EC, because he’s thinking, “Why do I need to go to college?” But, I’m impressed by him.