Definitely the mosaics, as Erdosian notes. I wasn’t sure these would be anything great, but found them to be absolutely excellent. Among them is the temple of Galla Placidia, built around 1600 years ago.
I went to Venice for five days in July and I was sorry to leave – I had a marvellous time. I just wandered around drinking it all in, going into the churches and so forth. Despite this being the same time as the Venice Biennale (which was cool in itself), I didn’t find myself particularly overcome by the hordes, either – maybe I just tuned them out.
You’ve gotten many good recommendations, especially from Götterfunken. I would add two. First, the vaporetto (water bus) is an attraction in itself, as far as I’m concerned – I spent plenty of time just cruising around on it, looking. You’ll be using it anyway to get around, it’s cheap (especially if you buy a pass), and it’s just about the most romantic public transit in the world. I didn’t even feel tempted to take a gondola ride – the vaporetti were really all I needed in that department.
Second, I didn’t go up the campanile in St. Mark’s Square; instead, I went across St. Mark’s Basin to San Giorgio Maggiore, the one with the similar-looking campanile facing St. Mark’s. The church is Palladio’s last work, but on a whim I took the elevator up the campanile – only a few euros – and the view was heartbreaking; the entire archipelago of Venice at your feet, and the horizon so vast the earth curves. I can’t describe it. I still remember how I felt.
The only stain on the whole experience was what a pain it was to eat. I was on a tight budget, and it seemed that nearly the only things that were reasonably priced were frozen and reheated. The only exception was the gelato - cheap and wonderful; if I could have lived on gelato, I’d have had it made.