I travel solo in Germany a lot and do not like a big sit down drawn out lunch. I go quick and light.

Most big cities have a Vapiano. I like them for a lunch or a pasta dinner. You order at the counter, then pick it up. Pay when done. Nice food.

Dean and David is another chain which has a lot of fresh lunch options. You order, pay, and pick up your food.

Sometimes I hit a Kebap shop in the main train station or in a central square. You can order it and eat it there, or find a seat in the square.

I also recommend the main train station for lunch. It is full of fresh, cheap, quick options.