Austria and Bavaria are a bit far from Cologne, if you are looking for something closer, consider doing the Rhineland area.
The historic city of Aachen is rather close to Cologne.
The Trier, Germany (an ancient Roman city) and Luxembourg are interesting. Also, the scenic Rhine between Cologne and Frankfurt. Rudesheim is great for taking a short river cruise.
If you choose to do Brugges, you could also, take in some great WWII battle sites. Also, Ghent and Brussels. Amsterdam is great as well.