I still recommend driving, but make sure you consider tolls in the calculation, in addition to gas. (We drove from the Dolomites to Venice and then from Venice to Germany). As an example, the toll from Venice back to the Austrian border was well over 20 Euros. I don't think you will save on a partial days' rental, especially not combining with the train.
We were in a different part of the Dolomites (by Brunico), and we stopped in Bolzano and then passed through Verona. Until we got to Bolzano, we took a surface road (at first on accident and then decided to continue) that ran parallel to the Autostrada. It was a lovely little drive, and we stopped to get fresh fruit from a merchant in a truck on the side of the road. What a memory!