Keep in mind the weather and hours of daylight at that time of year. It's usually not particularly cold, but rather damp and overcast. This casts sort of grey hazy curtain over the countryside. The atmosphere won't negatively affect most of the urban destinations you have picked, but it does mean that instead of the brilliant technicolor vistas you see in travel videos (usually filmed in the summer), the rural landscapes hide behind a layers of fog and haze for most of the day, on the majority of days.
Also, you may want to supplement your research with other sources besides Rick Steves. His book concentrates predominantly on southern Germany, and he tends to elevates some destinations as being far more unique than they actually are. For example, there's about 100 officially designated spa towns in Germany, but reading his book, you'd think Baden-Baden was the only one. If you really want the bathing experience, much closer to Düsseldorf are the ancient city of Aachen, Bad Neuenahr just off the Rhine (it's twin town, Ahrweiler, maintains it's old defensive wall and many of it's traditional buildings), Wiesbaden (the king of spa resorts) and probably several closer to Düsseldorf that I don't know about. Look for a guidebook that devotes some chapters to the states of Nordrhein-Westpfalen and Niedersachsen.
The famous Mittelrhein is within daytrip distance of Düsseldorf, but it's a little far. You may want to consider spending the night somwhere along the river so that you don't feel rushed by the setting sun. Also, don't overlook Bonn, which is a very elegant city.
I wouldn't bother with the Black Forest. At that time of year, it could very well be hidden in fog. If you really want to see a forested mountain range, the Eifel, Sauerland, Westerwald, Taunus, Hunsrück, Odenwald and Pfälzerwald are all much closer. But as noted, early winter simply isn't a good time for exploring the wilderness.
There's plenty to do in Düsseldorf for a shopper or business traveler, less so for a leisure traveler. However, the upscale residential district of Oberkasel on the opposite bank or the Rhine can offer a nice urban walk. I don't believe there's any particular "sites" to explore, but rather it's just a very pleasent neighborhood filled with amazing turn of the century architecture.
If you'll have a car, consider a daytrip to the small Belgian city of Hasselt. The city itself is pleasent, if nothing special, but every year during the holiday season, they host "Winterland", which is sort of an elaborate hybrid of a carnival and Christmas market. Best experienced in the evening.
