The Salzkammergut (literally translated: Property of the Salt Chamber) is one of the most beautiful holiday regions in Austria, with its name coming from its history. As the premier spot for Salt ("Salz") Mining. As salt was and is important it was under direct control by the Emperors in Vienna - that's where the name "Kammer" came from.
If you have seen the movie The Sound Of Music then you know what that lake region looks like, because that movie was filmed in and around the city of Salzburg and the neighboring Salzkammergut region.
Sparkling clean lakes, green hills, wonderful mountains, romantic towns and cities (like St.Wolfgang and Hallstatt make up this magical region that is filled to the rim with scenic beauty and a rich local culture.
If you ever visit the city of Salzburg and don't see this glorious lake region just half an hour by car east of the city then you have truly missed out on the most beautiful part of Austria.
The regional train has a comprehensive service running through the heart of the Salzkammergut and on into Styria. This provides easy access to the region from both Salzburg and Vienna, passing through Gmunden, Bad Ischl, Bad Goisern, Hallstatt and Obertraun.
I would however suggest car hire as the best way in which to explore this fascinating region. This gives you the freedom to explore area such as Gosausee and St Wolfgangsee, not accessible by train.
Skiing on the extensive Dachstein West Ski Region - 140km of Intermediate Piste + the Krippenstein Freeride Arena, for those looking for an exilhorating off-piste experience 
Swimming in the lakes
Walking on the Gmunder Promenade
Visit Hallstatt a UNESCO World Heritage site, a stunningly beautiful village.
The Dachstein Ice Caves at Obertraun, are well worth a visit. From from 1st June until 30th September 2012, travelers can buy a UNESCO World Heritage ticket  which costs 31 Euro and can be used for the ice cave and a boat trip across the Lake Hallstatt.
Mountain Bike, there are many trails for all levels of biker with stunning scenery around every corner.
The Krippenstein is a stunning paragliding site, rising 1500m from the valley floor.