Spent four days there this June and enjoyed it immensely. I was prepared to dislike St. Mark's Square, but I found it to be the most beautiful urban space I have ever seen -- st. Mark's Basilica, Doge's Palace, Bridge of Sighs, etc.. Favorite quote about the square: Napoleon -- "the drawing room of Europe." I'd spend most of a day there, which I will do when I return.
As always, I would get a local guide to help you walk around the city for most of a day. We walked great distances, and only got lost a couple of times Venice is not very big -- and there are NO vehicles present -- all deliveries are by boats, which make their way down very narrow waterways.
Check NY Times travel section from six months ago on adventurous dining in and around Venice.
By all means, go to Peggy Guggehheim's museum. Also, the rather plain-on-the-outside 14th C. church, Santa Maria Gloriosa, is stuffed full of Titians, but be sure to drop bread crumbs when you visit, so you can find your way back.
Traveling the Grand Canal is a treat, if the crowds on the vaporettos aren't too great. "Gondola rides" are just a historical curiosity -- take one for 45 minutes or an hour.
We stayed near the train station, but on returning, I would stay closer to St. Mark's.
Sage Grouse
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'When I got on the bus for my first road game at Duke, I saw that every player was carrying textbooks or laptops. I coached in the SEC for 25 years, and I had never seen that before, not even once.' - David Cutcliffe to Duke alumni in Washington, DC, June 2013