I spent 3 years working in a diagnostic radiologist's office. What you are going to be able to see really depends on the practice. In my office, all modalities were represented. In a day, the radiologist would read ~70 plane films (arms, broken fingers, etc), 10-15 mamograms, 10-20 CTs, 10-15 MRI, 3-5 flouroscopy cases (barium swallows, Intravenous Pyeograms, Upper/Lower GIs, etc), 10 or so ultrasounds, and several nuclear medicine studies.
If you've never experienced radiology, all of these things would be interesting - especially if the radiologist takes time to tell you what you're looking at (cause you won't have a clue, especially on ultrasound). If you've been around this stuff before, seen a chest xray, seen a head CT, and you're looking to see invasive stuff, you want to aim for the flouroscopy studies. VERY neat! It involves real time capture of radioactively labeled fluids traversing various vessels (ureters, intestines, etc.). It takes a lot of skill on the radiologists part and you, as a person who knows nothing, will be able to make out the basic anatomy and understand what the goal is.
Now if you're really lucky, you'll get to shadow an interventional radiologist. They are almost entirely procedure oriented, wheras a diagnostic radiologist is mostly interpretation oriented (read: sits in a dark room for 9 hours a day).
In general, it can be hit or miss. Totally depends on the number of physicians in the practice, the case load, and the willingness of a radiologist to take time to show you interesting things (most radiologists are VERY busy).
Anyway, that's about all I can think of right now. If I think of anything else, I'll post it.
-Stick