I guess I am lucky, they threw me right into the ER, I went through no orientation, they just said, "you look like you have a good head on your shoulders: to the ER with you!!!"
Still it isn't that much different, it isn't the craziest ER with traumas 24-7, just a steady stream of old people and panicky mothers with their healthy babies. What I do like is talking to the docs who most of the time will try to explain things to you (though there are some who just sit their stone cold, they are stunning). It is also fun to watch all the politics a doc has to deal with, seriously it is on so many levels, they get it from the nurses, the patients, other doctors, the administration, the lab, relatives....
You won't learn much medical stuff volunteering at the hospital; you are honestly relegated to supporting roles. -- My dreams of the doctor being short handed for a code and yelling, "Nathan get in here now!! Help me save this man's life" have been dashed. -- What you will get out of it is an understanding of the hospital environment, the politics involved, and what makes a good doctor (I always watch how the doctors and nurses interact with patients). It is an invaluable experience if you approach it the right way; you get out what you put in.