When addressing someone always use the formal Sie until you are begged by them to switch to the informal du. When in doubt, shake hands; both as a greeting and at parting.
It's polite to ask if someone speaks English (Sprechen Sie Englisch?) before addressing them in English.
Germans have done a lot of soul searching as a nation as to their role in and responsibility for the atrocities of World War II. They tend to be very well informed about world affairs—and often know more about the minute workings of the American political process than Americans do. Discussions about history and politics will almost always make for an interesting evening.