These are my suggestions when travelling around Italy, and these are in no particular order or importance,
* on the top of things to see is the "David " sculpture in Florence(of course by Michelangelo)
*print out city(walking) maps off the net of all the places you'll be on foot Rome, Florence, Venice
*book a stopover for eg: 6 hours in Dubai, Singapore (you will feel tons better for it)
*book guided tours for the big galleries (in Florence) and when visiting the Colloseum, Roman
ruins- this enables you to bypass the line ups and they are always interesting
*when visiting the Vatican you must have clothing to cover your shoulders and knees(they will refuse you entry)
*pack a single strapped backpack for all your walks (it's easier to protect your valuables than a double)
*when travelling on eurorail, purchase a first class ticket- it's worth it!!!
*take a camera with a quick recovery time (taking shots on the train)
*choose your accommodation in Venice wisely (i found it expensive & disappointing)
*try the local village wine as it doesn't contain preservatives (light & healthier)
*stock up on munchies so in the middle of the day for when the shops are closed
*paying for the use of toilets is commonplace (if there is a Macdonalds - they are free to use)
*when using a ferry in Venice - don't bother buying a ticket, as they are way to crowded to check if anyone has a ticket