Quite briefly: Is there anything better than cooling off in a clear mountain lake after a day in the mountains?!
Okay, so no one later says we didn't warn you: Since the lake is fed by mountain water, it's correspondingly cold, but as you know, only the tough ones get into the garden ;) The water quality is, of course, top-notch.
If you're more of a "coldblood" type and only dip your toe in the water at most, you can of course make yourself comfortable on the large sunbathing area - or you can rent a pedal boat and enjoy the view of the mountains with dry feet (well, hopefully :P )
And if even that's too risky for you and you really want to play it safe when it comes to water contact: There's also a circular hiking trail that can be combined with other longer hikes.
If you get hungry, a barbecue area or several places to stop for refreshments await you.
And if you need another tip: diving is also very popular here. Especially in autumn, the chances of good visibility underwater are really good ;)