All you want, when it is hot, is some form of water to jump in and cool down.
We jumped on our bikes and after a couple minutes we settled down next to the Almkanal and jumped in.
The Almkanal is one of Europe's oldest water systems(connecting a stream with the main river and bringing water to the city) going back all the way to the 8th century. The water is freezing and has such a strong current that you can't swim against it.
The thing to do is walk up a bit, jump in and swim or drift all the way down to where you want to get out.
We had the best time swimming, swinging across and jumping in the water, playing kubb, reading and relaxing.