Hey Andy,
I am from Chicago, so I am certainly used to the cold weather. Although, I have already found Düsseldorf to be milder than Chicago. Weather will likely be 25-30 farenheit and cloudy; that seems to be the status quo around here. As for suggestions on what to do, there are a few:
- Certainly hit up the Altstadt (Old Town) - full of restaurants and bars (250 of them...at least that is the published number).
- Drink some Alt bier...the local brew.
- Check out the shops around the Altstadt and Königsalle.
- Go to the Christmas Market (weinachtsmart) and have some Glühwein (spiced, hot red wine).
- If you have time, take the train down to Cologne (only 25 min away) and check out their Christmas Market and the city center/dome.
Also, apparently they have guided "Alt Bier" tours around Düsseldorf...might be worth checking out. Although, I am not really sure how good the tour is:
http://www.duesseldorf-tourismus.de/article_en2.php?
Hope you enjoy!
Cheers,
Chris