Best Swiss city to visit with tween/teens?

Did you actually swim in the river at Bern? We were going to do that but when I looked up information on where to start etc. this article was at the top of the search results:

https://www.lebendige-traditionen.ch/tradition/en/home/traditions/aare-swimming-in-bern.html#:~:text=Despite%20clearly%20visible%20entry%20and,the%20tradition%20to%20their%20children.

Despite clearly visible entry and exit points, the Aare is dangerous and only suitable for confident river swimmers.

That gave us doubt and we decided not to try it. So instead we walked along the river on the path to see it done. We saw a woman miss the last exit ladder and people started yelling at her to get out ( there is some kind of waterfall downstream from there). She made it to the bank but had to climb up over the rough rocks.

I would not advise sending kids into the river unless you go with them snd clearly understand how and where to exit.

It is easy to scroll past the warnings at the top of the page to the attractive photos and miss the point—- that the things pictured, like “river swimming is fun for all ages” are MISTAKES.

Then you see the red button at upper right for Official River Rules and the page is in German. Look at the menu on the right to find English. The next page is titled Danger to Life! In red. But 4 of the 6 rules are about boats. The other 2 address the dangers of open water swimming ( like rivers) and cold water, but those are generic and apply everywhere.

Nothing specific to swimming in the river at Bern and its particular hazards. So please look further for information and advice
before mentioning this to the kids and getting their hopes up.

If you want to really show the kids a true European city, I'd be tempted to go somewhere like Paris, but that's definitely obviously a more involved choice and with only 3 days probably doesn't make sense.

In terms of staying in Switzerland, Montreux would be nice. We stayed in a village along the water (Saint-Prex) which was nice because we had direct access to swimming in the lake, next to the boat dock for trips along the water and 5 minute walk to the train to go into the "city" to see things like the castle Chateau de Chillon, etc.

Another option could be staying in Luzern where you have access to day trips into Bern, Zurich, etc., as well as water activities, and then are very close to the airport for departure.

If you haven't gotten lodging yet, you're going to want to get it ASAP. August is high season and a lot will be already booked.

Luzern. It's got mountains and lakes and enough city for everyone. Just over an hour by train from the airport rail station in Zurich.

Another vote for Lucerne. Beautiful scenery, plenty to do. Good base for a day trip to Mt Rigi.