you can take the train (
www.db.de) directly from dus airport to cologne main station (hauptbanhof) in 40 minutes
dus is a commercial city, the most interesting things are the konigsalee (main shopping street-but you can see similar things in other large cities) and the altstadt- old city, with restaurants and boutique-y shops. fine museums, too
the hotel scene is poor- the steigenberger is old and ratty
the crowne plaza is, i think, the best in downtown and walkable everywhere
there is a maritim at the airport which is convenient if you have an early flite
cologne is an old roman city- ruins, history, and a giant cathedral- more interesting, i think, than dus
but beware the hotel ernst- in the shadows of the cathedral- the bonging of the gongs will keep you up all nite
each city has its local brands and types of beer- like kolsch or weissbier - just ask what the locals drink
and beware of schweinhaxe!- it's a platter of a huge pigs foot with sauerkraut and stuff- it sits in your stomach for days- not good for traveling
whatever you do, make sure you are not in either city when there is a "messe" or trade fair- the cities will be jammed