After Barcelona, Venice is my favourite city, as a non-driver, I love that it's largely a pedestrian city and once you figure out the boat system, its easy to get about in leaps.
The city just exudes history and style,
One word or warning though, last time I was there was in Sept, on one of the islands, the mosquitoes ate me alive, not a problem at this time of year.
I know a couple of Venetians! They are both very opinionated about Venice and how to travel there and more importantly how NOT to travel there. Both strongly suggested (with Venetian there is no other kind) not to go there in summer. Specifically they say to avoid late May to October and carnival!