Welcome to flyertalk
akelzz. Will you have your base in Cologne and do only want to do day trips or are you more flexible? What are you interested in?
General suggestions are:
- Visit the cathedral in Cologne and one of the many museums in Cologne, for example the Roman/Germanic museum or the chocolate museum
- Trip to Bonn to visit the birthplace of Beethoven
- Trip to Amsterdam
- Trip to Aachen
- Trip to Essen to visit one of the industrial museums
- Trip up the rhine valley