I feel that volunteering at a hospital is very similar to how AdComs want to see shadowing involved. Volunteering at a hospital is your shot to see how the hospital operates, from unit to unit observations to, security, transport, EMS, ancillary, administrative and the such. Once you get into the hospital setting, you'll get that exposure, but you won't have the freedom to really explore the way that a volunteer has.
Just what I think about it. Volunteering at a hospital will take a while since they do have to get medical clearance for you. If you have a clinical/semi-clinical exposure, then I'm sure a non-clinical volunteer experience would suffice. I know that for NYC residents there's something called NYCares, and they offer volunteer position placements, if your state/city offers it, then definitely check it out.