Venice Travel Guide (Venice, Italy)

British citizens can enter Italy and stay as long as they like on production of a full passport. Similarly unrestricted access is granted to all EU nationals, whereas citizens of the United States, Canada, Australia and New Zealand are limited to stays of three months, though they, too, need only a valid passport. All other nationals should consult the relevant embassies about visa requirements.

Legally, you're required to register with the police within three days of entering Italy. This will be done for you if you're staying in a hotel (this is why you have to surrender your passport on arrival), but if you're on a self-catering trip you should register at the Questura (HQ of the state police). It used to be the case that nobody bothered too much about this formality, but in recent years the police have begun to be more pedantic with backpacking types in Venice. So if you think you might look like the sort of person a Venetian policeman might deem undesirable, get registered.