Only 30 minutes north by car there is a lake that looks more than Sweden than Waldviertel in Lower Austria. Due to its elevation temperatures are significantly cooler at lake Ottenstein. So when it gets hot in the Wachau valley we recommend to take a day trip to Ottenstein to swim, take a boat, SUP, read a book in the shade of the pine trees...
Maybe you would want to combine this day trip with a visit of the Loisium in Langenlois.