Here are ones I've shadowed during high school/undergrad:
Orthopaedic Surgeons: Very cool procedures (hip replacement, hammer toe, knee replacement). Usually fairly quick surgeries and fairly easy to see what's going on.
Interventional Radiologist/Cardiologist: Pretty much watched them do angioplasty/stents all day. They were really nice and would take a few minutes here and there between cases pointing out things as they were reviewing ECGs and other images. For the actual procedures you'll probably be staying in the "control room" watching on the monitors there. May see some stress test stuff. I would think with a normal cardiologist you'll see more clinical/office stuff and see more doctor-patient interaction.
ENT: Was a fairly large private office/surgery practice. The Doc was real nice and saw a good mix of clinical visits as well as sinus surgeries (mostly kids).
Emergency Med: Probably my favorite. LOTS of doctor-patient interaction and saw a lot of different stuff.... cool/nasty infections, trauma, general sickies, pysch patients, minor procedures, etc. Went several times and saw a lot, everything from the normal non-emergency "my stomach hurts", to motorcycle accident victim with foot hanging off. This hospital was a large teaching hospital so I pretty much spent all my time with the residents, but they were real nice, and since they were not much older than me, we talked about a lot of non-medical stuff.
I also got extremely lucky and thanks to that particular Doc, I rode/flew with the local Life Flight (Helicopter rescue) crew on a few calls, amazing!
Anyway, just see what you're interested in. I second the idea of mixing it up by spending some time with surgeons, and then more clinical or primary care type specialties.
When you do go pay attention to how much paperwork can be involved, I know it surprised me the first couple times.
