Ring: Chlodwigplatz to Ebertplatz. Lots to do and see in this semi-circular area that follows the edges of the city's wall. This is the place where there will be life on the street even late at night, at least on the part north of Rudolfplatz (the Hohenzollernring - all the segments of the Ring have different names, but they all end in "-ring"). There are many cinemas and clubs (of the posh variety) on the Ring, off Friesenplatz to the east you will find lots of bars on Friesenstrasse and if you feel like a late night (or early morning) snack, the area around Friesenplatz is the place to look for it. Otherwise, there are of course the usual burger chain restaurants, lately joined by a few upmarket places serving traditionally American, Italian and Mexican food, that will satisfy your needs. However, you should definitely consider turning just one or two more corners into the Belgisches Viertel in order to find food of the same quality at far lower prices than on the Ring (rents there are obviously high). Here are some neighborhoods listed south to north along the Ring:

Südstadt: Great area for good cocktails and good music, particularly rock. Two theatres are located in this neighborhood: the Theater am Sachsenring and the Theater der Keller.

Uni-Viertel: Major draw for young people and college students. Good bars, pubs, and clubs.

Friesenviertel: a more expensive and elegant area where media moguls and artists often congregate.

Belgisches Viertel: the area between Hohenzollernring and the park (Grüngürtel), which frames it from the west, extends to the south up to Lindenstrasse and to the north up to Venloer Strasse. So called because most of the streets are named after cities in Belgium or the Netherlands, this is a mixed neighbourhood which draws the young and hip and also the hip, not-so-young, but affluent. There is a number of interesting restaurants offering foreign cuisine and even more bars to be found here. Also the area is very good for shopping for ready-to-wear tailor-made couture in nice boutiques, which tend to the pricey side.
In summer nights of the past, the residents of the buildings around Brüsseler Platz had to put up with a yearly growing amount of noise from the crowd spontaneously gathering there to enjoy their sundowners and a few more drinks in the square surrounding the old church. These days, it has become much tamer, as gastronomy has filled the space available to the masses and a public urinal has been installed in order to preserve the flower beds that the residents lovingly take care of.

Nordstadt:
more cinemas and less bars and restaurants, however, you should be able to find something to eat. You can get (mostly Turkish, and delicious) food until very late at night in the area around the Eigelsteintor (located a few hundred feet north of Cologne's central station) - some of the restaurants are open 24 hours. This is one of the seedier parts of the Nordstadt, it needs to be said, though you needn't worry about your safety even late at night. In some of the bars (and not only shady ones) you may find a relaxed coexistence of the residents of the area and the s*x workers who are looking to meet customers. The square in front of Eigelsteintor is a popular hangout to enjoy the ice cream from the surrounding ltalian parlours. It is a good idea to keep an eye on your belongings though and to always lock bikes before leaving them out of sight, even if only for a short minute.

Nippes: off the beaten track but very interesting area. You'll be glad you sought it out.

Ehrenfeld: removed from the inner city by Cologne's green lung, the Grüngürtel, this colorful revitalized area in the west of Cologne is full of young people and lots of activity. The mosque of Cologne, a rather impressive building in bright concrete, presides over the intersection of Venloer and Innere Kanalstraße, which limits Ehrenfeld from the east. Find a variety of bargain ethnic dining options encompassing Turkish, Japanese, Greek, Mexican and more down Ehrenfeld's main east-west artery, Venloer Strasse. Fantastic aromatic bread shops are located on each street corner. Explore the narrow side streets leading off Venloer to the north between Innerer Kanalstraße and Ehrenfeldgürtel to find friendly little shops selling friendly little things like toys, children's clothes, decoration items, textile products, fair trade coffee and other things needed to make a house a home. Numerous bars and small clubs (don't worry about not coming in with sneakers) are located in this area, many of them west of Ehrenfeldgürtel, Ehrenfeld's main road dividing it from north to south.