Primarily, you might want to visit Alps around Salzburg for mountains themselves - whether you're into climbing, trekking or just taking cable line all the way up. That's something we did in 2008 for several days and then tired we sought refuge at the lakes. And that was truly the best part...
We started off at Hallstatter See, the most southern one, close to great trekking trails. It's kind of wild when compared to others, less frequented, yet amazingly charming. The lakeside town of Hallstatt itself is a true gem of typical alpine architecture and pleasing wealth of bygone centuries - a major Celtic archæological landmark and a UNESCO World Heritage Site. There are two campsites at the the lake - smaller
Then we took the road no. 145 up north to Bad Ischl and west towards much more splendid Wolfgangsee. It takes his name from ultra chic town of St. Wolfgang im Salzkammergut - much bigger and expensive than Hallstatt. If you fancy to get snobbish, than that's definitely the place, though accommodation doesn't come cheap. Fortunately, on the opposite, southern side there much more "relaxed" area where we managed to find very decent camping and gorgeous access to water with a floating deck. Misty morning there with quiet bows rowing in the distance eases up any stress.
Next on the itinerary was Mondsee, much smaller lake up north, which you can approach straight from A1 motorway (there is even beautiful lookout and parking for those who just need a peek). On the southern coast there are numerous public beaches where everybody can stop and rest, and swimming here is truly gorgeous with mild sand on the bottom and intimate little bays.
Finally, we made it east to Attersee, my personal absolute favorite. It's not only the largest lake in the region (20 km long) but has best water ever, thanks to limestone bottom. The colour seems to be totally photoshopped at all times and the temperature is always pleasantly refreshing. It's a great sailing place with legendary steady winds, including the rose-scented one - that's aroma breezed from the rose garden at the castle on the island Litzlberg, where Gustav Klimt used to live during summers. There are at least three very good campings at the lake, with many German people having their trailers settled for the whole season - our favorite being Camping Grabner at the village Steinbach. It's a great base for short trip to Burggrabenklamm, an utterly scenic waterfall right at the lake (20 minute hike from parking - you need good shoes!).