Unless you get a late start, Linz is a little too close to Vienna for lunch. What I saw of it I wouldn't call quaint, either. Its nice, but a fairly large city.
I have not visited Dürnstein, but it's even closer to Vienna.
If you take the more southerly route, Salzburg might be around the halfway point, considering that traffic will slow down considerably once you cross the border into Germany (the A8 along this section is very scenic, but also extremely congested). Bad Reichenhall is a nice little spa resort town right over the border, if you would prefer not to drive into a larger city like Salzburg. Further along the route, Rosenheim is a pleasant town, but by the time you reached here, it would likely be quite a bit later than lunchtime. This route also takes you by Priem am Chiemsee, but this (and Herrenchiemsee island) would likely make a better daytrip from Munich than a stop-over.
If you took a more northerly route, you could stop off in Burghausen. The quaint old section of town here isn't very big, but it boasts one of the world's largest castles.
I haven't visited, but your route also takes you directly by Melk abbey, which is even visible from the Autobahn.