All of this advice is really sound--and congrats to you for doing such serious planning ahead!
I just want to reiterate what others have mentioned: foreign language. It's super important, and it's the one thing you can do now that might actually decrease the amount of work you'll have to do as a ph.d. student. I'd suggest continuing in German, since you have a background already, and perhaps trying to take some beginning french (or spanish, if you prefer) classes, if you have room in your schedule.
Getting involved in the departmental events is also a great idea; not only will you see great films/speakers, you'll strenghten the relationships you have with professors (your future recommenders), you will also get a bit of insight into how these events are connected to the all-over workings of the department.
Two things you might also want to do: try to present a paper in an undergraduate conference. We had ungergrad conferences at my school in the spring, and I had to present at one as part of a requirement for a course (I hate speaking publically, so I probably wouldn't have done it otherwise). It's a great experience, allows you to gain some knowlege of how to field questions from the audience, see how panels are put together, etc. The other thing you may want to do is take a history course in your area. Since you're more or less interested in 20th century american lit, it may be a good idea to take a course that is focused on ww I and ww II, the depression, labor movements, and so on.
Lastly, it's awesome that you've already tried to narrow down your interests, but just remember that there is a big chance your interests may change. Try to take a variety of coursework. And I think theory is extremely important (though I'm a bit biased, I suppose): if you do decide to change your field, a strong foundation in theory will help you do that without losing depth to your interests. Good luck!