As Rodgers has already mentioned, most Germans know at least some English from school, and in my personal experience, are eager to try it out. It helps if you speak slowly and without too much an accent to be understood.
Though if you try German, the Schwaben - people living in the Stuttgart area - can be hard to understand with their very thick accent. (Even for Germans from other areas.)
Topics to avoid: the war and Georg Bush, if you are a supporter - most Germans are against.
Cultural differences to remember: Germans are much more relaxed about sex, but very adverse to violence re movies, PC games etc.
Southern Germans - mostly the Bavarians, but maybe also some of the Schwabens - are also not as friendly as Americans might be used to, e.g. when you look at somebody, they don’t smile back, they look away. This doesn’t mean they are unfriendly, they just don’t smile all the time.
Germans also tend not to chat casually with people - they look for deep, real friendships, so if you talk for hours on the train, it may bother your opposite. Or it may not - depends on the circumstances.
Depending on what train you are taking, bring headphones - the more expensive ICE trains have radio on board.
Remember for any electronics you bring, you’ll need an adapter for the plugs AND we have 220 Volts, not 110, so you need a transformer or similar.
Cars: While it’s true there is no general speed limit on the Autobahn, that doesn’t mean you can drive 250 kmh everywhere - there are speed limits in towns and on the smaller roads. Some parts of the Autobahn also have speed limits (I think because of too many accidents there). Don’t drink and drive. When visiting any of the larger cities like Stuttgart, I strongly recommend the Public transport system - you don’t have the hassle of looking for a parking space, or trying to find streets in a foreign city and in a foreign language.
Speyer is a very old town, and Stuttgart, as capital of Baden-Württemberg, is also worth sight-seeing. I don’t know what kind of tours they have, though, but just ask at the tourist office.
Foods: I can’t reccommend all the meat dishes, as I’m vegetarian, but special foods in the Schwaben region are Spätzle - noodle dough scraped into hot water - and Maultaschen - dough filled with meat and boiled in a soup. There’s also Schupfnudeln - noodles made from potatoe dough, and fried in a pan, sometimes together with Sauerkraut.
There are many traditional dishes, and they can differ in taste, so I recommend trying to see what you like.
Remember that we use metric, so distance and speed is in Kilometers (roughly a yard), drinks and food are in liters (roughly a quart) and kilos (roughly two pounds). Currency is the Euro, and the different values have different colours and size (some Americans think the colour means it’s monopoly money. It’s not, it’s for the people with bad eyes to make distinguishing easier). 1 Euro is very roughly 1 Dollar. The cent coins (100 cents = 1 Euro) have different backsides according to which country issued them; many people like to collect them, maybe you would like, too.
Remember to bring enough money or traveller cheques, because withdrawing money can carry high fees. Remember also, if you want to withdraw money at the ATM, that a PIN with more than 4 digits likely won’t be accepted. Visa may not be accepted everywhere. The most common card is the EC-card, which is linked to a bank account (debit card?), not a credit card. There is much more cash payment for small amounts, credit cards are for big purchases (because of the fees for the merchants, many have signs that they will accept credit cards only above a certain amount.)
If you buy big souvenirs, you can get the VAT (Mehrwertsteuer, currently 16%) back with a special form. The big shops have service desks for this. (The MwST is the same for the whole country, no special sales tax.)