I think you'd need to do a night in Salzburg. You wouldn't get much out of a day trip.

While I haven't been to the Melk Abbey, it's supposed to be quite interesting, and it's on my list for next time, hopefully this year. Melk is a more reasonable day trip from Vienna.

Bratislava is also a short bus or train ride away, and another good day trip. Go in the morning, wander through Old Town, have some lunch, maybe find the Blue Church, then head back to Vienna.

Otherwise, I could easily find 5 days of things to do without leaving Vienna!