I'm an American living in Germany so maybe I can help, hehe.
Just curious but what parts of Germany will you be going to?
Americans and Germans are different but not enough to the point where it's likely you'll offend somebody with your day to day normal actions. If something offends them I'm sure they'll understand, but here are a few tips:
Don't bring up WWII, but that should be a no brainer.
Don't bring up American politics cuz Europeans hate Bush and they love Obama. So if you aren't a fan of Obama, steer clear of politics.
Attempting to learn the language is always makes you look good, no matter what country you go to. Especially if you try to order food or a drink. It'll probably score some points with the ladies too.
If you are visiting someones house I'd say it'd be nice to bring a gift. It's cool if you bring something from where you live like I know for sure old German men like shot glasses. When I went home on vacation I brought some shot glasses back with Mississippi stuff on them.
When you go out to eat the tip is usually part of the check so you don't have to leave money on the table. If you really want to leave a tip give it directly to that person.
Some Europeans have a different sense of personal space than we do. They tend to get closer when they talk to you than we would. So don't overreact and be like "hey man back up". I live in the German countryside and the people here are very friendly towards the American community. If i meet new people they usually by me drinks(cuz i'm the lone American usually) and the tend to get real close and put their arm around me like we're brothers. That's just how they are sometimes so I wouldn't take offense and think they are trying something.
If you go out and drink with Germans and you happen to be an angry drunk who likes to fight, then I suggest that you don't drink cuz they look down on that. Germans can drink twice as much as Americans and can keep their composure twice as good.
I can't really think of anything else. Those tips I gave you are from my experience from living in Rheinland Pfalz. Every part of Germany is completely different from the next. From the language, the culture and the overall mood of the locals. Germans from the south typically like to remind everyone how awesome it is to be from the south much to the chagrin of everyone else. Up north people can be very cold at times and in the backwoods of the country people can be warm and welcoming and everywhere else it's inbetween or to either extreme, it's a very diverse place. But Germany is an awesome country, and I know you'll have a great time and I'm sure you won't offend anybody.
p.s. contrary to American pop culture, Germans are not huge fans of David Hasselhoff.