Limit your time in Cologne, add more time in Brugge. Stay in Brugge at least for 1 night to get the evening atmosphere. I would add that Brugge is worth one night more than Brussels...
Don't worry for the chocolat museum in Cologne. You'll find another one in Brugge, (http://www.choco-story.be/) plus all the chocolate shops you'll find in town, some making chocolate behind a window inside the shop.
Have a nice trip.

I found a day in Brugge enough, same with Brussels, but Amsterdam and Paris
are great cities ; one can always find more to see and do.

I agree on extending the Bruges sidetrip and I would miss out Brussels (where there is only the Manekin Piss to see)

Bruges on the other hand has so much from the Chapel of the Holy Blood (reputeed relic of Christ) the Michelangelo statue of Madonna in the Vhurch of our Lady (2nd tallest brick structure in the world) etc etc. I go there every month and there is hardly a country in the world I haven't been to. Bruges is my very favourite.

Amsterdam is also fun - the dutch very friendly - museums good red light area touristy and interesting!!

Have a great time. Try this link for Belgium Bruges information. Enjoy!!
http://www.oldchocolatehouse.com/Links.html