St. (Lake) Wolfgang

I don't know who told you that Hallstatt is five hours (even round trip) from Salzburg. Viamichelin lists it at 1 hour, 11 minutes one way. St Wolfgang is pretty, too. When I was a student in Austria, we did Mondsee, St Wolfgang, and Hallstatt as a day trip from Salzburg, so you could do both, conceivably.

As Nancy says, driving to Hallstatt is faster than by train. If you were going by train instead of driving, it's 2hrs 20min from Salzburg to Hallstatt, one way, thus x 2 = 4hrs 40 min roundtrip, and adding a bit for waiting for the train the roundtrip by train would be about 5 hrs.

For Austrians, St. Wolfgang is as picturesque a site as Hallstadt is for foreign tourists. You can reach the town easily by bus from Salzburg (but since it's across the lake from St. Gilgen, you need to find transportation to Strobl and then up to town. Be sure to take photos of St. Wolfgang from St. Gilgen. There also is ferry service if you want direct routing.
Once you get to St. Wolfgang: Views to the Alps are outstanding - the viewpoint is of the area Sound of Music was filmed in. The Schafberg behind is one of that region's tallest peaks and can be reached via gondola.

St. Wolfgang's famous and romantic White Horse Inn (of musical renown) sits next to the dominant church and promenade. Check bus routes in Salzburg or at www.salzkammergut.co.at

Thanks for all your input. I guess I forgot to mention we are going strictly by public transportation, hence the five hour round trip by train to Hallstatt. Any idea how to connect with the ferry? Where does it leave from?

Connection with ferry is at Abersee, about halfway toward Strobl. Check www.salzkammergut.at