Hi
I know Venice very well and run workshops there most months. September will be busy and still packed with hordes of tourists so I strongly suggest you get into the quieter areas.
My workshops focus on the areas where the locals live, work and play. You still get the pretty bridges, the gondolas, the reflections etc but not the selfie-stick brigade. I would recommend the districts of Cannaregio, Dorsoduro and the western part of Castello. I would completely avoid the tourist traps like San Marco and Rialto - you won't get an uncluttered shot anywhere.
Consider early in the morning (as the sun comes up) - shooting from San Zaccharia towards S. Giorgio with the Gondolas in the foreground - it should be very quiet then and you'd get a great atmospheric shot. Do the same from Academia Bridge. Also consider hiring a water taxi (expensive, but I guess the clients would pay) to do a George and Amal Clooney type shot - the taxi pilots are very amenable to this and you could get some terrific shots in the early evening light along the Grand Canal.
Venice is all about light and if you look for it you'll find amazing light everywhere. But don't obsess too much about the exact location; my advice is to visit the above areas and just walk - you'll find surprises around every corner.
Hope this helps,
Brian
