This is a nice city to visit when you have a day or two. Beautiful old buildings and very lively people. Large Japanese minority. Dusseldorf is easily accessed by air, train or bus and is just around the corner from Cologne.
Favourite spots:
Zeil (city centre), the Rhine vicinity. Ask Dusseldorfers for more, they are very friendly people.
What's really great:
Openness of the people. Good museums. Japanese food. Alt beer
Hangouts:
Anything along the Zeil; check with young people for latest clubs
From my experience I know there are plenty cities in Germany worth a day or two, provided you use this time wisely. More details about the museums and historic monuments as well as accommodation would come in handy.