Hi guys. I currently am doing the medical career exploration program at brigham. It's probably the best volunteering opportunity you can get for your time. You have to commit 6 months minimum to the program and volunteer at least 3-4 hours per week. After 110 hours, you get a great letter of rec sent out with your med school apps. Basically, you spend a few weeks doing tons of different things in every different department in the hospital. You shadow some of the best docs in the northeast and get to see everything the field of medicine has to offer.
For me the program just made sense. I am an electrical engineering major and had thoughts of medical school. Before making such a huge commitment to medicine (time-wise and financially), I wanted to make sure it was the right choice, therefore I joined this program. The second I started I just had the strongest feelings for the field and now cannot see myself doing anything but going to medical school. I have also witnessed the opposite, where some in the program realized they didn't like the sight of people dying and being sick. Sometime you really don't realize that until you've seen it day in and day out first hand.
I believe MGH has something similar, but cannot say for sure. Go for brigham though. It's the biggest of the Harvard teaching hospitals and who knows, you may find some kind of connection. Go in there with an open mind. Most people are known to have "tunnel vision" and often can't accept the fact that they really don't like medicine all that much. The best part of the program is that you'll actually enjoy your time at the hospital instead of just doing it because you have to.
P.S. The hospitals in Boston are ridiculously huge. I got lost the first 50 times I had to work around the L1 and L2 floors at brigham and women's.