Switzerland Thur Chessel

Big stones in the river, deep blue water (at least after the snow melts in spring)

It's not really depending on the weather, but take care of heavy rain in the mountains.

Combine with hiking of the Thurweg. You can take the Bus/Train at Krummenau or Nesslau Neu St.Johann. Or park there, walk up or down and take the bus or train back to your car.
If you walk up in direction of Nesslau Neu St.Johann there is the «Inseli» of Johanneum, a beautiful and nice prepared river island where you are allowed to grill. You can even just use the wood and infrastructure there.