My husband is going to a conference in March in Dusseldorf and needs a place to stay. The conference is in Kaiserswerth, and he'd like to stay either very near to the conference or very near to the best parts of the city. Any suggestions are welcome!

The Orangerie is a beautiful boutique hotel in the old town, with an excellent yet quiet location for strolling through the city, nightlife etc. However, it is in the center of Düsseldorf, while Kaiserwerth is an incorporated suburb.
http://www.hotel-orangerie-mcs.de/

There are few hotels directly in Kaiserswerth,however nothing special:

http://www.villa-fiore.de/
http://www.hotel-kaiserswerth.de/
http://www.ambrosius-hotel.de/

The best and newest hotel is the Intercontinental, it's right in the glitzy Alee area.

famous luxury hotel The Steingeberger parkhotel. Just the end of the Konigs allee THE upscale shopping avenue. Close to everything tthey have special 2 night shopping or we packages etc. Just great.

excuse me typo: Steigenberger !