The Solarbad at Dorfgastein is our 'go to' place on a hot day. We usually go off for a hike in the morning and then go to the Solarbad to cool off. The kids love coming here and spend most of their time on the 2 slides. There's a warm water area for playing and a cold water area for swimmers.
The Naturerlebnis Dorfgastein is also very close, with a Kneippanlage, a fantastic play park, and a nice walk through the woods.