Dusseldorf is famuos for various types of exhibitions / business fairs. I also happened to visit the city for the same reason. Although my visit was quite a short one due to tight schedule, I still managed to take out time for a stroll at Alstadt. Also there are a number of parks and probably the one that you might wish to visit is the Konigsallee in the centre of the city. Here the two sides of the street are separated by a canal and green spaces on either side.
Dusseldorf is a nice city none the less. If you have to come here for any reason then it is worth taking time out to explore. Especially, a night out in the Altstadt has much to recommend it. So many bars and restaurants to choose . When I visited Alstadt, it was a crowded like mad. People drinking dancing enjoying. It was amazing weather.
Düsseldorf is at its best along the Rhine: from Oberkasseler Rheinwiese opposite the Old Town or the Heerdter Rheinschleife, you can get a particularly nice view of the city silhouette. For a wide view right up to the Ruhr Valley, take a car to the eastern part of Düsseldorf.