Try taking the ferry from Italy to Greece. They travel from Bari, Ancona, Brindisi, or Venice Italy. To Corfu, Igoumenitsa, or Patras and vice versa. All ferries are atleast one night and there are different companies for each route with different times. If you have a Eurail pass it will cover part or all of your ticket, but will only give a ticket for the deck which means no bed or room, so be prepared.

I'm originally italian so I'd like to share a few important tips for backpacking this magic country.
1) transportation in italy works pretty bad, often train arrived late so plan in advance.
2) don't skip the south and don't be scared of trying to get to know italians, but beware lots of people will try just to get as many money as possible from you (taxi drivers first)
3) bring a dictionary:) not a lot of italian will be able to efficiently speak with you in english (especially in the south)
4) visit the main touristic site if you want to but if you have time don't skip, Calabria, Sicilia and Piemonte to see some more traditional italian culture.
let me know if you need a tip, there's so much to say:)