If you are travelling between Vienna and Salzburg and have a little time, you may want to make a break and see some of the interesting places along the way. Your options are of course much wider if you have a car, but many of the following places are also well accessible by train or bus (or boat).
So where to stop and what to see?
The Fastest Route
Being two major Austrian cities, Vienna and Salzburg are linked by the country’s most important motorway (A1) and railway corridor. The distance is slightly less than 300km.
Linz
The most convenient stop, especially when taking an express train or bus, is Linz, the third biggest city in Austria. It is very industrial and can’t compete with the charm and cultural abundance of either Vienna and Salzburg, but still offers plenty of interesting things to keep you busy for a day or two: a large old town, a castle, the biggest church in Austria, a zoo, or some of the best Botanical Gardens in Europe.
That said, as you are already visiting Vienna and Salzburg, you may prefer a smaller place as your stop.
Wachau Valley
Some 60km upstream from Vienna, still closer to Vienna than to Linz, the Danube flows through a picturesque hilly region, known for its wines, monasteries, and castle ruins – such as Dürnstein Castle, where English King Richard the Lion-Heart was held captive by Leopold V on the way home from the Third Crusade.
The Wachau Valley, a UNESCO World Heritage Site, is accessible by road or train from either Krems or Melk – the latter is better, as it lies on both the A1 and the main rail line between Vienna and Linz.
That said, the best way to explore the Wachau is on a Danube cruise. You may want to take a bus or train from Vienna to Krems, then take a ship to Melk (the upstream cruise takes 2-3 hours, opposite direction is faster), and then take a train from Melk to Linz or Salzburg. If you have a car and need to start and end the cruise in the same place, expect a full Wachau round trip to take 5.5 to 6 hours.
Mauthausen Memorial
Just before Linz, also on the Danube and by the A1 motorway, is the small town of Mauthausen, site of a large concentration camp during World War II, and now a memorial and museum.
Steyr
If you want to visit a typical smaller Austrian city, there is hardly a better choice than Steyr. It is situated 30km south of Linz at the foothills of the Alps and at the confluence of rivers Steyr and Enns (the latter enters the Danube in Mauthausen). Steyr has a well preserved old town centre with a castle, number of churches, monuments, and other historical buildings. It is not a particular thing that is a tourist magnet, but the combination of architecture and natural surroundings make a complete picture that has something very Austrian to it.
If you are driving, Steyr is just a short (20km) detour from the A1. By train it is accessible via St Valentin (on the Vienna-Linz main line).
Salzkammergut Lakes
If you want to taste the real Austrian Alps, leave your stop to just before Salzburg. There is a large area known as Salzkammergut, full of lakes, mountains, waterfalls, small towns and villages. You can actually see some of it directly from the A1 motorway (the railway line passes a bit further away in the north). If you are driving and have limited time, stop at the Mondsee service station, whose parking place feels like a balcony overlooking a lake with mountains in the background (Mondsee means Moon Lake).
If you have more time, you can leave the motorway at km 216 (Laakirchen) for Gmunden and Traunsee, and possibly continue through the lake district all the way to Salzburg, visiting Bad Ischl, Wolfgangsee, St Gilgen, and Fuschlsee.
A shorter version is to leave the A1 at km 234 (Seewalchen) for Attersee and return to the motorway at Mondsee (km 264).