Evenings. I volunteer 5 PM to 9 PM on a Wednesday and I get to see almost everything. Just last week, there were two MedFlights that came in during my shift. Wow! Also, I got to see a REAL emergency; this guy came in, almost couldn't breath and was huffing and puffing, was having a severe allergic reaction coupled with severe asthma. Great stuff, but you need to volunteer long enough to get the cool/interesting cases.
And its not just the cases. Depending on where you are, volunteers can get away with helping patients with other mundane stuff. Today I helped a woman use a telephone, helped a guy get his jacket on and walk him around, ran to get patient charts et cetera. There's lots to do, but only if you actually seek it out. Otherwise, people are too busy to deal with a clueless volunteer.