An overnighter in Innsbruck is fine. This city may not have the attraction for some travelers, but it is a historic capital (of Tirol), and apart from the medieval town center has other attractions - Emperor Maximilian is buried in the cathedral, so visit the elaborate memorial, there is an Olympic Museum to commemorate several Winter Olympics there, the Berg Igls site famous for the Olympics as well as Tirolian history, the Svarovski factory in Wattens (no factory tour but one heck of an artistic must-see), and there are all the mountains. We love the Stubai Valley and always stay at the Neustifter Hof (Neustift). The glacier gondola is not far away.
Salzburg - well-loved by all tourists and worth at least 2 days (although the major sites can be visited in one). Stay outside of Salzburg, if you can. Bus transport is great and lodging cheaper. I'd recommend the Gschirnwirt in Eugendorf. See Rick's book for many other ideas in the Salzkammergut area! We'll see you there in July.