Grundlsee itself is so lovely. But a quite cold lake, to be honest.
Bad Ausse, Altaussee, Bad Ischl and, obviously, Hallstatt. Don't leave Austria without having seen at least Hallstatt: www.hallstatt.net/home-en-US/
Take as well time for relaxing. That's why I on purpose don't write to visit asd well the city of Salzburg.
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As said above the area itself is lovely.
Don't miss out on Salzburg.
Bad Ischl has the "Kaiser Villa" of Franz-Joseph and Sissi. There's also the residence of the composer Lehar, with a stunning wooden door at the interior.
The lakes in the Salzkammergut area are very beautiful.
We also love Gosau. There's a funicular that takes you up the mountain from where you can do several walks. From here you have a great view at the Dachstein glacier. You can also walk around the lake Gosau or take a road that leads somewhat further than the top of the lake.