You can (and MUST) rent a costume at a recommended atelier but you have to buy your own mask. Ateliers don't rent masks for safety reasons. Besides, you'll want to keep your mask forever. And rent that costume as early as possible- they're all one of...
If you're a high roller, it's definitely the Doge Ball. Just google it and you'll see. If you're doing a once in a lifetime trip, then go to the official ball, which has a different theme (and therefore a different name) each year. And there are a do...
Stay somewhere pretty. Don't skimp on ambiance in Venice. Then get up early one day to beat the crowds to Florian. After 9am, you'll wait an hour in line. Worth it.
Libreria. Photographer. Doge. Enough said.