Hello,
One great thing I did was become an Eagle Scout; I learned so much, made so many friends, and helped out a local zoo. That is an accomplishment that I will proudly take to my medical school application in May.
I also shadowed my ENT for a few days. That was an excellent experience. I would highly recommend finding a doctor to shadow as this will tell you if this is really what you want to do.
Volunteering is great too! I didn't do any hospital-related volunteering just yet, but in general, it is a great way to give back to your community!
Most importantly, get good grades and learn as much as you can! Take advantage of as many AP/IB/College level courses as possible as this will give you a head start when you get to a university. Also, I earned a 4 on my AP Chem exam which gave me credit for an intro chem class at UW-Madison, which some (?) medical schools will accept as satisfying one of two gen chem course prerequisites.
Stay motivated! I know it seems like you're a lifetime away from applying, but hang in there. I felt that way for a long time and still do, even with my application cycle beginning next year!
Good luck!