Ermmm... Cologne is actually one of the most modern cities in Germany! I have been there. The entire city was burnt down some time ago, save for the cathedral, so they rebuilt it and hence it is new. A good place for shopping... it's something like the Tokyo of Germany... lots of modern hi-end shops... also interesting shops for collectibles stuff. Good for street photography (pretty high human traffic)... not really for scenery. Unless streets are scenic to you. But yes, the train ride to Cologne crosses the Rhine river, which is very scenic.
The most famous thing in this city is the cathedral, which is said to be the only thing that didn't get burnt down in the fire. It is one of the oldest cathedrals around... very large... very grand... very historic/ nostalgic looking architecture.... with lots of intricate carvings... greyish in colour... kinda gothic like. I love the architecture... it's one of the most beautiful cathedrals I have seen (but then this is my taste). Worth a visit, esp. if architecture is your thing. There are free (i think) conducted tours in the cathedral too and you can light a candle if you like. Have fun!
Oh, you can probably skip Frankfurt... pretty boring commercial city compared to the others...
I know a German lady from Munich... I would ask her for advise, then get back to you.