for snacks, almost every bakery has a few quick sandwich type things. they are usually very good but not as cheap as they used to be. they are the german version of american deli sandwiches.
my personal favorite grocery store is aldi. you must bring your own bag or pay for one, and they only take cash, but everything is very cheap. they have a good selection of picnic type foods.
in every town there are also konditorei, which are like a coffee shop but usually much more fancy. they are not cheap, but they are fun for an elegant dessert in the afternoon.
and there are imbiss and kebab stands. and pizza by the slice. and unfortunately subway, burger king, and mc donalds.
we always stay where we get breakfast in our room (coffee is very expensive in germany, by the cup, no refills) and eat a nice, big breakfast, have a glass of wine or beer with a snack around lunch time, then we usually have a nice dinner. for us, the food is important; we are past the souvenir stage