Dusseldorf vs Konigswinter as a day trip from Cologne in June

What time of year?

Sorry, in June. I thought I had that included in my original post.

Königswinter and Drachenfels plus a boatrip on the Rhine from Cologne might be nice.

Depends of what you prefer another city trip or more small town and relaxed boattrip and a great view from Drachenfels

Vote for Königswinter.

We visited Cologne for 3 nights in the winter for the Christmas markets, which were fabulous. It wouldn’t be my choice along the Rhine for general sightseeing. The city was heavily bombed, and not much old-world charm remains, except the cathedral. The city is modern and bustling. Aachen would be an easy day trip to a smaller town with more charm.

I much prefer the area from Koblenz to Bacharach in the summer. Boppard is a great base. Tons of charming towns along the Rhine, tons of castles. I would not allow train tickets to dictate where you spend your time, given the significant investment in getting there.

Just stumbled across this video of a day trip to Konigswinter.
https://www.youtube.com/watch?v=bJ7EtIBxMSw

June 1, 2025 will be the monthly "Fishmarket" along the Rhine in Dusseldorf. Trust me, wine, beer, sausages and Oma's Quarkbollen are more important than fish! I really enjoyed one, some years ago.

https://www.rheinlust.de/veranstaltungen-2/dusseldorf/

Dusseldorf is particularly a place to see modern and contemporary art, especially because of the world-influencing Kunstakademie Dusseldorf. But there is at least one nice museum of ... dead artists there. And an incredible number of brewery restaurants. It's a long U-Bahn ride, but I enjoyed the large Botanical Garden.

I presume you considered Monschau and Wuppertal as well.