We drove from Salzburg to Vienna last September. The weather was so bad, heavy rain most of the way, that I cannot be of any help as all I saw were road, cars and rain.
However we stayed (in a forgettable B&B) near the Wachau Valley for a couple of nights and spent one day doing a river boat trip on the Danube from Krems to Melk and back. The Wachau Valley is reputed to be the prettiest part of the Danube and it certainly compares well with the Mosel and Rhine. Lots of churches, castles and, of course, Melk Abbey. Melk Abbey and its gardens are worth a half day stop. Try to time your Abbey trip for the English tour. You can see a brief version of Melk and the Wachau Valley tour by Rick Steves on Youtube.
While in Salzburg take a day and tour the Salzkammergut and Hallstatt.