Breweries
Malzmühle - one of the less obvious breweries on the southern side of Heumarkt next to Maritim Hotel; not extremely beautiful but very original with lots of traditional dishes; be sure to also try the 'Malzbier', a sweet malt beer with very low alcohol content.
Brauerei Paeffgen - located on Friesenstrasse 64-66, this is the place to go when you want to get some original food between shopping and night entertainment. Some clubs are right next door in a street that is closed for traffic at night. When it's warm enough, take a stroll down Venloer Strasse and go to 'Stadtgarten', a beer garden with Jazz bar and night club.
Früh and Gaffel - probably the best known breweries in Cologne, located at the Cathedral on Roncalliplatz and on Alter Markt. These also serve very good food (if you like it) and have a more pleasant interior.

Fast Food
Cologne (and Germany, for that matter) has a lot of Turkish restaurants. The most famous dish is the Doener Kebab which has become a German favorite, too. Especially at night you will notice hords of people prefering it over McDonald's specialties... Always choose the garlic sauce to experience the real deal and have it put in the bread before anything else!

Keupstrasse - in the district of 'Muehlheim' near Wiener Platz this street has almost only Turkish shops and restaurants and is located next to some major tv-studios. Two of the biggest German late night shows are being recorded here as well as the programme for a music channel. Just pick any restaurant in the street and enjoy one of the best Doeners in Germany, maybe have some Turkish sweets in a bakery afterwards. I have never had a problem in this street, but maybe you should go there during the day at first and pick up the vibe.

Kyffhäuserstrasse - on the corner of Kyffhäuserstrasse and Heinsbergstrasse there is a very good Doener place. They will bake the bread for it fresh with every order! It's also in the heart of the students' party district and close to major subway/tram lines and taxi stands.

Then of course, there is the sausage ('Wurst')! You cannot go to Germany and not have a 'Currywurst'! In my opinion, it is much better in Berlin but people from North Rhine-Westphalia often (and untruly , check my username) claim to have invented it. So, after having sampled quite a number I can recommend a booth on a tiny square where Ehrenstrasse and Breite Strasse meet. You can only stand there to eat, but it's right in the middle of the best shopping area, very easy to fit in the day's schedule. I suggest having a 'Krakauer' sausage (that is the red one) prepared as a Currywurst...

I have to stop here, because it's gotten rather late. I hope this is helpful for a visit to Cologne.