If you do not want to walk all the way to Cologne, you can also take a small tour with the Cologne Bimmelbahn. This is not only a great experience, you can win the city once from a completely different Perspaktive.
There are two routes. One goes to the Cologne Zoo and the other along the many sights in the city center. This takes you past the famous chocolate museum through the old town and along the Cologne Cathedral. A really great route.