I question peanut butter also. But I will say that my German foreign exchange students did like the Reeses Peanut butter cups. They liked them alot.

My boyfriend and I are usually ask for
Reeses Peanut butter cups
M&M's with almonds
Hazelnut Belgian Instant Coffee by Kraft
Maccaroni & Cheese - I especially like the ones from Annie's
when friends come over from the U.S. or somebody is going there.

To say that you can get everything you could get in America anywhere in Europe is completely false. There are American products in the international section of many supermarkets, but they tend to be high priced, stereotyped and not particularly high quality (Heins or generic barbecue sauce, off label macaroni and cheese, spray cheese (!?!), peanut butter with tons of sugar and preservatives). There might not be a market for many American products in Germany, but that's largely the result of ignorance, different tastes (I'd mostly put peanut butter in this category), or the expense of shipping things from one developed country to another. I've had great success bringing food items back with me from the US for gifts. Nearly anything from Trader Joes is usually good, jars of salsa, real New York bagels, sharp New York state or Vermont cheddar, tortillas, the previous suggestion of a can of clam chowder was a really good one, pumpkin pie filling (make the crust from scratch), rice cakes, anything with cranberries (e.g. tea), hummus and pita bread (I'd kill for some and can't find it here), cans of spicy pinto beans (Germans are crazy about making chili con carne, but never have the right ingredients), soy milk, beef jerky. I'm sure there's plenty I'm not thinking of since I haven't been in the US in a long time.

Thanks for everyone's suggestions! Now I am armed with some really excellent ideas!Also makes me feel like I have taken for granted things like reeses peanut butter cups and mac n' cheese since it's so readily available over here.
I am definitely going to go to trader joe's tomorrow and stock up on annie's mac n' cheese (i love that too), reeses peanut butter cups, some clam chowder and other delicious things. I can't wait to see my friends and share the goodies with them. Thank you all so much!