Hot springs or public baths around Cologne/Gelsenkirchen?

not very far from Cologne is the town of Euskirchen which has the Therme Euskirchen https://www.badewelt-euskirchen.de/en/

I have visited the therme and enjoyed it. It is part of a small chain of places that do a good job and provide a nice experience.

The Palmenparadies is a bathing suit wearing fun space - nice water, nice splashy water toys, good atmosphere, very clean.

There is a sauna area which is in a different part of the complex, in there suits are forbidden for hygiene reasons and for the same reasons there must be a towel between skin and bench. It is an extra fee and your daughter wouldn't be allowed - minimum age is 16.

Just by the way - you do know that Cologne and Gelsenkirchen are not near each other don't you? Gelsenkirchen is way up near Dortmund...

Thank you Nigel! We plan to spend a night in cologne and 1-2 nights in gelsenkirchen. I see that the train trip between them is 60-90 min, will be prepared for that.

It is probably a bit far South, but in Bad Ems, near Koblenz, is Emser-Therme, a big modern baths, suits in the main area, but nude in the Sauna area (which you will find everywhere I suppose).

It would be a bit far from Cologne (2.5 to 3 hours) to go just for that, but if you were planning on some time on the Rhine, it might be a stop.

Edit: Slipped my mind, but closer to Cologne, would be Aachen (45 Min to 1 hour). A neat town itself, and the Carolus Therme there may be what you are looking for.

I love Carolus Therme and have visited many times. But for a first timer there aren't many water toys and the water can be slightly cool sometimes. For regular bathers passing by it is very good. I don't remember ever seeing young people there, in fact many are older than me and I'm older than dirt.

There was another poster taking their daughter (a bit older than 14) to Gelsenkirchen to see Taylor Swift. If you 2 are interested in mid 20th century Industrial architecture, the UNESCO Zollverein mine and coking Museum is nearby. Otherwise Gelsenkirchen is probably good for one night after the concert. 2 nights in Cologne would be better.

Amsterdam is a pretty easy train ride from Gelsenkirchen with a single train change in Oberhausen. Less than 3 hours over all.

looks like there are several to choose from - https://www.google.com/maps/search/therme+near+Gelsenkirchen/@51.5114517,6.6987734,10z/data=!3m1!4b1

I'm a bit of an aficionado. I only write about those places I have actually been to, so I'm afraid that anything north of Cologne is unknown to me.

We plan to spend a night in cologne and 1-2 nights in gelsenkirchen. I
see that the train trip between them is 60-90 min, will be prepared
for that

Not a bad idea, Cologne is a nice city worth a stop, but Aachen can as mentioned also be a good option. Regarding the train to Gelsenkirchen, there are a number of stations in the town so check which one is closest to where you are going. It is in the middle of the Ruhr area where it's often hard to see where one town ends and the other begins, so the closest station might actually be in another town.

Dusseldorf might also be an alternative to staying in Gelsenkirchen, only 45 minutes or so travel time. Rooms in "Swifty" cities have been filling up fast.