The Hohenzollernbrücke is probably the most central climbing spot with the best view in a big German city. You can climb the basement of the bridge directly at the Rhine with a view on the famous Cologne cathedral.
Of course, climbing the 70 routes itself isn’t too exciting as only top-rope is possible. But the location is worth it. Difficulties range from 6 to 7 UIAA most of the time. Don’t forget to bring a long sling so you can install the top-rope. There are some access issues and rules, details can be found on the webpage of the DAV.
After the session, you can get beers at the Hyatt hotel right next to the spot and enjoy the boulevard.
Climbing Hohenzollernbrücke? These people have collected all the information for you.