You just have to go to the "Altstadt" (old city). There are over 200 bars, clubs and restaurants around the "Bolkerstraße". I'm sure that you will find a perfect place to stay. There're will be not so much party like on weekends, but it's still okay.

The Bolkerstraße is between the "Königsallee" and the Rhine and you can find it easily when you take the Underground to the station "Heinrich-Heine-allee/Altstadt"