Berchtesgaden is just over the border, although it wouldn't make much sense to pop over the border just to say you were there. It deserves a full day on it's own.
The direct driving route from Salzburg to Villach takes you directly by Burg Hohenwerfen. I never had time to stop for a visit, but it looks magnificent from the road.
You also pass close to Radstadt, which is featured as a worthwile stop in several guidebooks... and I'm not sure exactly why. It's pretty enough, certainly, but didn't strike me as anything particularly special. Oh well, perhaps there's something I missed.
The Autobahn route mostly skims through the valleys. If you want to drive a detour over a high pass, near Radstadt, take the secondary road up to Obertauern. I love this town, but it's a modern high altitude mountain resort, not a quaint rustic village. So, probably not worth stopping to explore the town itself, but the scenery of the mountains will blow you away. Continue the drive back down the pass to Mauterndorf, and you can pick up the Autobahn again nearby.