I would fly into Vienna on 12th May (early morning). Plan to stay there for 2 days & then rent a car & drive west to Innsbruck stopping on the way in Hallstatt for couple of nights.
I was wondering what would be a good option for a centrally located hotel in Vienna next to a car rental agency so that I can easily pick up a car on the morning of my departure. The other option off course could be to pick up a car when I land from the airport but then I would be paying for 2 extra days for no reason - since I don't want to drive inside Vienna city.