The same as any hospital, really. It depends on where you volunteer and what service you're in. I volunteered at one of the clinics and usually did stuff like bring patients in, take vitals, clear examination rooms, transfer specimens, etc. I knew a guy who volunteered at ophthalmology and didn't do much, unfortunately. I also knew a guy who volunteered at the ER, as well (you don't do as much yourself, but you probably see a lot more).
(Source: I volunteered at the VA, did clinical research with the VA/the nearby university, and now I work at the VA on the research floor)