St. Helens sounds like a fabulous idea. I'm a pretty big geological nerd when it comes to things like that. Of course, like everything else, it depends on the time we have.
Although now I hear that Yellowstone itself is pretty jumpin' and more potentially dangerous these days. (All the more reason to visit) :-D
Gen, actually we'd stay near traintrack-laden hotels on purpose, because my two best friends are total train nerds. I like 'em too, but they're out of control. :) Thanks for the pointers. I'm rather looking forward to that 4 Corners dealie.