What's there to see and do in Koln (other than the Cathedral)?

I was only there for one day but, yes the cathedral was the main attraction, and if you've seen a bunch of cathedrals . . . .

We ate at one of the many outdoor cafes on the main square and enjoyed the view, and a fresh Kölsch.

I love Cologne, which is a beautiful city with a lot of ambience and the most gorgeous cathedral you will ever see. There are a number of good restaurants and also has some fun shopping as well. One restaurant I recommend is maiBeck FÜR DICH, which had some of the best food I ate in Germany. I also ate at Brauhaus früh am Dom, which serves authentic German food. I opted for beer and sauerbraten, and it was very tasty. Both restaurants were within walking distance of the cathedral.

Since you're staying for a bit, that will give you time to explore Cologne and all of it sights, but will also give you time for a day trip to Aachen, which is a quick 30 minute train ride away, and well worth a visit. Its cathedral is quite beautiful (especially the interior, which is drop-dead gorgeous) and houses the remains of Charlemagne, leader of the Holy Roman Empire. The town is also very charming and has a lovely vibe to it. It's smaller than Cologne and you could easily spend some time just walking around and enjoying the sites. Don't miss the Granus tower, which is next to the Rathaus (town hall) and is one of the few pieces remaining from Charlemagne’s palace. It dates back to the 8th century, making it over 1200 years old.

Much more, three days at least.

https://community.ricksteves.com/travel-forum/germany/can-i-see-enough-of-cologne

Chocolate "museum"

Roman museum

Walk along the river

Discover what makes a Kölsch uniquely a Kölsch

Cable car over the river

Zoo

Discover what the difference is on your dressing table between a plain Eau de Cologne and Kölnisch Wasser 4711

Make an effort to understand the local dialect

Take a short train ride to Königswinter and ride the Drachenfelsbahn, the oldest cogwheel railway in Germany https://www.drachenfelsbahn.de/en/ up the hill to the ruined castle https://www.drachenfelsbahn.de/en/drachenfels , enjoy the view of the Seven Mountains, have a bite, enjoy all the other things up there.

Take a short ride the other way to Aachen and discover why Charlemagne was so interested in it (there is a nice Therme there too if you want a couple of hours getting warm and wet).

We were originally going to stay in Cologne for 2 nights in July but chose Dusseldorf instead. But we will visit Cologne on a day trip to see the cathedral and the Lindt chocolate museum. Already eyeing Brauhaus FRÜH am Dom, Peters Brauhaus or Brauerei zur Malzmühle are breweries to hit for lunch.

You could use Cologne as a base and trip to Dusseldorf or Essen. Dusseldorf has Little Tokyo and a decent shopping scene. Many folks recommend Aachen as well.