As far as cities go, Cologne is a pretty good one. There's a lot of atmosphere down by the water with nice restaurants and a couple of museums. It was busy without being over crowded, even around the major attractions. I visited the Nazi museum housed in the old Gestapo headquarters and I could not recommend it enough for anyone with an interest in the period of history. I suggest definitely taking a walk over the bridge that leads to/from the main train station to see the thousands of locks.
Insider Tip:Take an approx. 2 hour train to Cochem and hire a bike to ride to Eltz Castle. A beautiful ride along a river and hills covered in vines, it is about 20km each way but completely flat and relaxing. It is then a 2.5km walk through lovely forest to reach the amazing castle. Cochem also had a lovely castle, Reichsburg Castle. It was a fantastic day, and I cannot recommend it enough for anyone with an interest in the outdoors and castles.