The Cathedral of Cologne is the most visited attraction in Germany. It is built in gothic style and the landmark of the city, directly located at the main station and the Rhine.
The Romanesque churches are UNESCO world cultural heritage.
You also may want to visit one of Cologne’s many museums. There are all types of museums (e.g., art, history, and archeology).
For general information use the following link. You will find detailed information about almost everything that is worth visiting.