If you are staying in Cologne what other major cities can you visit in a day?

Bonn where Beethoven was born/lived, our capital when we were two German countrys (1948 - 1989)
D眉sseldorf, expensive shopping, lots of Japanese restaurants
river Rhine valley, take a cruise boot and visit a castle
Aachen Charles the Great's capital in the years arround 800, his Throne, Crown and many more things.
one day is a bit to short to visit all the interesting places arounf or in Cologne

http://gogermany.about.com/od/regionalva...

I stayed in the southern region of the Netherlands - Maastricht to be exact - and I went to Cologne and Brussels from there. There is a lot of stuff to see and do in that area. Where Netherlands, Germany and Belgium meet there is a spot where you can sit in all three countries and a cool Labyrinth in the same area. There is also an American Cemetery from WWII near Maastricht. Also, a train ride to Amsterdam wouldn't take too long - a few hours.

By the way, be sure to check out the Cologne-Bonn Cathedral there. It is a spectacular sight.

Well one fun thing is to take the boat round trip to Bonn, it's fun and they serve beer on the boat...It's a shortish day trip..Just go down to the docks to get your ticket..You will like it.. (Unfortunately Bonn (dispite great efforts) has never recovered from the relocation of the capital, but the University area is really nice.)

Ex capital from Germany BONN, or you go nice shopping to Dusseldorf.

You can go to Bonn, and possibly Dusseldorf, also in 2 days you might be able to go to Amsterdam, Holland

Wuppertal!!! :o)
That's my hometown and it's the only city where the trains hang upside down! (Suspension train)