I grew up just south of Cologne & have been there more times I can count. My first band practiced and performed mostly in Cologne 
So, I’m happy to share some firsthand knowledge from an actual native 
If you want a traditional Köln experience, you should def get thee to one of the many Kölsch breweries. Yeah, they can be touristy, but it’s a fun experience nonetheless. A lot of them are close to the train station (and the stunning cathedral), i.e. in the center of the old city, so you wouldn’t have to venture very far, depending on where you are staying.
Brauerei Zur Malzmühle even saw former prez Clinton darken their door and enjoying a nice meal of Sauerbraten, a very traditional dish from the area.
Sion is nice, too. The staff can be a bit abrasive, but it’s part of a shtick — and not just for tourists. Brauhaus Reissdorf brews one of my favorite Kölsch & has good, hearty food.
Some specialties to try:
Halver Hahn literally means half a rooster, but it’s actually a thick slice of gouda on rye with onion and paprika — leaving many a customer disappointed 
Himmel un Ääd, which is mashed & apple sauce, most often served along fried blood sausage or liverwurst
Sauerbraten (marinated beef)
Bratwurst is a no-brainer almost anywhere in Germany, and you’ll find your fill at the various xmas markets, too. Same with Riefkooche, which are like hashbrowns/latkes, usually topped with apple sauce.
The xmas markets have something to munch on for any taste (tho I personally eschew Glühwein bc I don’t care for hot booze of any kind, altho I’ll make the occasional excuse for mead, which can also be found at some xmas markets): garlicky mushroom skillets, Dampfnudeln (big fat yeasty dumplings with plum filling and vanilla sauce & poppyseeds), roasted chestnuts, brats, all kinds of sweets…
You won’t go hungry.
I’ve only been to Nürnberg once & spent but an afternoon in Strasbourg as a wee one, so I can’t really give you any reliable information for either place that would be up-to-date, and never have been to Zurich. Best to repost on the regional boards (France & Switzerland, respectively).
Gute Reise & Guten Appetit!