I've volunteered in the Pediatric ER, Adult ER, and the ICU.
The patients in the ICU are often very interesting cases and the doctors aren't as frantic and busy as the ER physicians, so they talk, to you more. However, there are some sad cases (i.e. Teenagers who have OD'ed in suicide attempts, elderly patients with no family to visit them). I found that in the ICU I was able to take part in more than in the ER or Peds ER, mainly because of the volume of patients.
The ER is really, really, REALLY busy. However, (at least for me), there's next to nothing that volunteers are allowed to do, so there's a *LOT* of time. I ended up finding a medical textbook and just reading sections that interested me, which was more interesting than waiting for the next person who needed a glass of water. The ER can also be rather depressing (at least in the NYC hospital I volunteered at). A segment of ER patients are there to find a warm bed and food, and there's nothing that anyone can do to change that. I suppose that's what being a doctor is like, though. I felt that even though I felt really down after volunteering, I learned a lot, and the experience strengthened my resolve to go into medicine. Being unable to do anything truly helpful made me realize how much I wanted to be able to actually help people.
Peds ER is fun, but the cases there are usually not really very interesting. In general, parents err on the side of overattentive to their children's health, and so the kids there aren't really horribly ill. The staff is really friendly though and, of course, you get to play with kids. I ended up actually being given a relatively large amount of work to do, but it was really repetitive. The doctors and nurses are really close, too.
I think the main thing you can do to make your volunteering experience most pleasant is to ask doctors and nurses if you can help out with or even just watch procedures. A lot of doctors are also really happy to talk about their experiences with premed classes, medical school, choosing their specialty, etc.
Hope this provides some insight, and apologize if I sound a little full of myself.
-SBBunny