Start with what's meaningful to you
Explore a variety of activities
Commit passionately to a select few
Few aspects of the admissions process tend to fray the nerves like pulling together an activity list in your senior year and reviewing your extracurricular pursuits. Almost no one feels they compare well with their peers. This feeling is only compounded by articles about other applicants taking an 8-week jaunt to Bolivia and doing ground breaking research on some strange virus, or playing with the New York Symphony in Beijing before hiking up the face of K2. These applicants do, probably, exist. But, they shouldn't deter you from taking pride in whatever it is you enjoy and excel in. When you're contemplating how you'll spend your time outside of class, keep in mind the golden rule: "What you do matters far less than how you do it." (p.64, The Thinking Parent's Guide to College Admissions by Eva Ostrum, Penguin, New York, 2006).
What's of utmost importance to you is finding activities that you really like to do. This is difficult because few of us really take the time (or, more honestly, have the time) to explore what's available. There are a lot of avenues to explore:
School clubs: MUN, student council, sports, school newspapers
Activities outside of school: internships, community service, sports clubs, church activities;
Work: web designing, retail, clerical assistance;
Hobbies: art projects, computer programming, basket weaving... The list of possibilities is only limited by your imagination.
The best way to discover what really primes your passion is to jump in and try things. I attended a conference in downtown LA, and an admissions officer from Princeton mentioned that when he was in high school, he believed he was destined to become a lawyer. Then he took a summer and worked in a law office and discovered law was not what he wanted to pursue. Did he consider this a failure? Absolutely not: learning what it is you don't like is, many times, just as valuable as finding out things you do. Discovering your calling is a process fraught with dead ends and frustrations. That's the nature of learning and discovery. Don't think that admissions officers aren't aware of the imperfections we encounter when we seek to learn. They are, and they, generally, respect applicants that bring their soul searching efforts onto the pages of their applications.
What admissions officers don't want to see is a candidate who plays it safe. Specifically, adding activities, such as packaged and expensive leadership seminars at Stanford or MIT, during the summer, don't, generally, impress most savvy admissions officers. What they're looking for, in whatever extracurricular pursuit you bring to the table, is:
Level of Commitment: Did you dedicate at least 5 hours a week, on a consistent basis, to the activity?
Level or Participation: Did you take a leadership role? Are you someone who becomes absorbed in the activity and really contributes fully?
Level of Skill Achieved: If in sports, did you become all-state, or participate in a national tournament? Are you the first chair in the regional wind ensemble? Did you get an article published on a noted website about James Joyce's use of imagery in "The Dead"?
Passion: To what are you most committed?
These answers, in turn, tell your prospective schools who you are as an individual, as a member of a quartet or a team, as a family member, as an employee, or as part of club. Your activities form a pattern of the various roles you play on a day to day basis. Eventually they are, in essence, who you are, and what you will become. That's of most interest to admissions officers, and, of course, to you.