As you may already know, being a successful medical school applicant is a matter of jumping through a number of subjective and objective hoops, followed by a weeding out process to make sure you aren't a freak (the interview). ECs largely fall under the "subjective" hoop realm, though in my case the lines are blurred somewhat. To make it through, you must have volunteered, and you must have shadowed physicians or explored medicine sufficiently. I tend to think of these activities as EC's, though I admit I may not be in the majority opinion on that one.
I made it into med school (on my first try), and I had:
*a 2.9 my freshman year, with ever increasing grades thereafter, cumulating in a 3.72 overall gpa and two honors degrees.
*a 33 MCAT
*strong rec letters
*mediocre volunteer quantity
*good volunteer quality (I could articulate clearly how the volunteering affected me & how it contributed to my awareness of the correctness of my career choice in medicine.)
*terrible shadowing quantity
*decent shadowing quality
*zero clubs/ECs
*a handful of honor societies
*3 good research experiences
I help out with my school's pre-meds every now & then, and one of the most frequent questions that comes up is: 'how many ECs do I need?" My view is that you should have a small number (1-3) things that you are interested in and that you do well in. Being involved in 11 campus organizations isn't as impressive as being involved (deeply) with several. Attend the pre-med club stuff regularly, then pick one or two clubs that you are really interested in, and stick with them for several years. I want to work with people who have diverse interests, but not if that means that they don't know what they want. Many high-schoolers are over-extended in terms of clubs, sports, church, etc. The truth is, any @sshole with a pulse can attend the minimum number of meetings for 12 different clubs. Don't be a club-slut. Also, in college, you will probably find that there is too much to do and not enough time to do it in. I *never* studied in HS, studied rarely my first two years of college, a little more my third, and a hell of a lot the last two. Add that to 50-30 hrs/week of work + a social life, and the days just aren't long enough. Stick to a handful of organizations, and do more than just show up for meetings. Pick one (or two, if you have a lot of energy and are ambitious) clubs & take leadership roles in that club. If you come from a wealthy family, don't have to work while in school, and find your classes easy, then you can do more. However, being a solid student while being involved in only a couple of ECs is fine, as long as your volunteer & shadowing stuff is covered.
Best of luck!
ps: remember to enjoy your time in college! my partner rushed through school to be a dentist & now that he is a DMD, he wishes he had taken more time to kick back & enjoy undergrad more. Just don't get bad grades, don't get alcohol poisoning, and don't get any STDs or make any babies. 😀