Train to Venice! Never pass up a chance to go through the Alps by rail.
The route I would recommend is the one we did last summer. From Zurich, take a leisurely train ride to Chur. Or better yet, rent a car from Europcar in Zurich and drop it off at the Chur train station. Chur is a nice little town in the foothills of the Alps. Was once a Roman settlement, now the centre of Swiss wine country. Downtown is eminently walkable. We stayed at the Hotel Freieck and walked from there to the train station.
Next morning, take the Bernina Express from Chur to Tirano, Italy. The most spectacular scenery including the train passing over a VERY high viaduct, and later around a most interesting figure 8 spiral. Train leave Chur around 8:45 am and gets into Tirano around 1:00.
When you get off at the Swiss train station in Tirano, walk across the town square to the Italian train station. From there, take a regional train to Milan, which goes along the shores of beautiful Lake Como.
At Milan, connect to a regular intercity train to Venice. You should be there by 8:00 pm, in time for supper and some Amarone wine.
Enjoy!