I have something to say about quality of volunteer experience.
I'm not particularly against having "fillers". At least you would have gained some insight how a hospital/nursing ward/ER/etc. operates and how it feels like to be in there, just by spending 100+ hours doing nothing during your volunteering. Fillers are absolutely better than no volunteering, and it's a no-brainer.
Anyhow, you can enter only 15 entries of your extracurricular activities, including volunteering, publications, research, employment, etc. Of those fifteen, you are asked to pick three most important experiences and elaborate more on them. I'd like to encourage you to plan ahead how you'd like to put in ingredients to adorn your primary application.
Of course, if you end up having lots of fillers, you should be ready to explain to AdCom why you hopped here and there for volunteering rather than staying in one institution for a long time. Just be careful not to give AdComs a negative impression by having 10+ volunteering work, ~ 20 hours each, for example.
From my experience, I'd like to suggest that you experience a bad (excuse my poor vocab..) hospital as well as a good one.
I don't want to talk about the importance of doing your volunteering at a good (affluent, well-organized and whatnot) hospital because you already know why.
On the other hand, I think your experience at a bad one would be more valuable than you might expect. You will observe chaotic organization, unhappy staff, and incompetent floor management (and dull cafeteria food). You will be exposed to an unattractive portion of your dream career. Show your commitment despite that and explain why you still wish to accomplish your career goal. If you had a good experience at a good hospital, and one at a less competent hospital, you would be able to formulate your own insight towards your role as a physician and as someone who has to spend majority of your time in these settings.
I volunteered at a world-class cancer hospital and at a small community hospital located in a poor neighborhood; I learned so much by encountering patients in these two extremes, and I think I grew up personally during these experiences.
As a volunteer, you will not be in a position where you can judge the quality of medical practice provided by the hospital. Also, no matter where you are, your exposure to physicians will be limited. Instead, you will have lots of experience with patients, and I think this is the whole point of hospital volunteering.
Lastly, a bad experience is a good experience. You will see why when you interview with AdComs.