I went with a friend to Cologne on a day off. Otherwise we would just have a few hours in the afternoon on the day of our concert to see the city. We were scheduled to fly to Vienna after the concert that night.
Cologne is one of my favorite cities. It has a world-famous cathedral, the Rhine river, its own special beer (Koelsch) and way of serving it, lots of Roman ruins and friendly people.
I found a great coffee place there with lots of old espresso machines lining the shelves up high.
We climbed the tower stairs in the cathedral and were in the belfry when the bells chimed.
The concert hall in Cologne is fairly new and beautifully designed. It is underground, below a plaza that goes out to the Rhine. Skateboarding on the plaza is "verboten" during concerts because it can be heard in the hall. The hall is large, but gives an intimate feeling. The audience is close and visible while you play.
Our concert went well. Afterwards I had time to go to a very old brewpub (dating from the Middle Ages) for a quick dinner before getting on the plane.