Are there activities/organizations that impress highly selective colleges?
Stand out by doing something different. Too many students see what they think some students did to be successful and try to follow in their foot steps. Follow your interests and passions. Try something new, challenge yourself to do something creative and then reflect on what you have learned. There is no "magic" activity that will get you in, it is what you have learned about your self in the process.
There has been a lot of emphasis on "leadership" and "Community Service". These are both good things. But remember that being a leader does not necessarily mean you are the president of an organization. Many students are leaders just by being committed to a club or organization. Being a responsible member, helping others take charge of activities are both aspects of leadership.
Avoid, by all means, doing activities because they "look good" on your resume. Take a real interest in what you are doing and find ways to b committed. Also, avoid over participating. Students often feel that if they have a long list of activities they look impressive. Keep the number of activities or memberships to a minimum and be a conscientious member.