2 nights in Cologne enough, or 3 better?

[Cleaningfail]

We just want to see Christmas markets and maybe do a couple of museums. Flight times are rubbish… flying at 3:30pm arriving about 5pm ish on a Saturday then return flight home is 3pm ish on the Monday. No other flight times that day from the airport we want

You won’t even be there for 48 hours. I’d either go from another airport so you can be there longer, or choose somewhere else altogether.

We did a similar Friday - Sunday. It was more than enough time.

We went to cologne December 2017 - there are multiple Christmas markets - all quite small but different - I think we did 6 - didn't do them all
Note - many museums in Europe are shut on Mondays so if you have a must do plan for Sunday
We went Saturday am - Tuesday pm, went mainly for a concert on the Saturday
Stayed at nh collection köln mediapark - fab hotel - even better breakfast (with complementary breakfast Prosecco)
Would love to go back

It's doable. The city centre is quite compact and the roman museum/art museum are next to the cathedral. The main market next to the cathedral will be rammed on a Saturday night. I was actually scared the last time we went - better during the day. The others which are dotted about weren't too bad.

Get the train?