Short term rental
If you are looking for short and long term rental have look at www.e-rent.de and zweitwohnagentur
I can recommend Ashleys Garden in Düsseldorf near by the fair center.
A nice place to stay, near Mannesmannufer's Vodafone Group headquarter - a 4 minute walk! Reasonable breakfast. Small rooms if you plan to stay longer than one month.
Location, location, location!
Einfaches aber gutes Hotel in Laufentfernung zum NSN Office Völklinger Str. / MVTC - Simple but good hotel in walking distance to the NSN Office Völklinger Str. / MVTC.
Mercure DDorf City, a 5 minute walk from Hauptbahnhof. Nice place, good restaurant - plenty of stuff to do and see from there!! Subway - 5 mins
Design hotel with great furnitures in each rooms. Offers apartments with kitchenette. Close to the Schumacher Brewery and restaurant on Oststraße. Great breakfast!