I’m half-posting to help with my own suggestions, and half-posting to follow because I’ve only ever spent a day in Rome several years ago and can’t wait to go back 🙂
On our honeymoon this past October, we spent 3 nights in Venice, 2 in the Cinque Terre, and 4 in Florence. If you decide to include either of those places, I can offer some suggestions, but since you mentioned Venice, I think you should check this stuff out for sure:
- Palazzo Ducale (takes a bit of time to walk through the whole thing, but worth it)
- Basilica di San Marco — the inside is stupid gorgeous, but I found the crush of tourists to be pretty overwhelming. Definitely sneak upstairs to the musem, which I thought was the best part. Even if you’re not into all of the history (but seriously, you can stand *thisclose* to the original Horses of St Mark’s, I geeked out so hard I almost died!), you get to walk through the second floor of the basilica and get some amazing views that most of the tourists miss.
- The Frari Church — I am not a big art or church buff, but I found it really awe-inspiring to see so many beautiful Renaissance masterpieces in the setting they were designed for… you don’t really get that in a museum.
- L’Accademia museum
- Go up the Campanile for a city view and great pictures
- Leave some unplanned time for wandering/getting lost off the beaten path — Venice is very fun to get lost in, and the Dorsoduro neighborhood is great for this — and also some unplanned Grand Canal cruising
- Don’t forget to do a couple of the silly touristy things — we had fun enjoying an overpriced Aperol spritz on St. Mark’s Square while we listened to the dueling orchestras, we did a sunset gondola ride, etc.
Definitely get a vaporetto pass if you’ll be in Venice for a couple of days — they pay for themselves quickly! Also, Venice is not big on amazing food because (a) tourists and (b) island, but we had the best luck just following our noses, not guidebooks. We found some real hidden gems! (this place stands out in my memory as being pretty fantastic and affordable: http://www.tripadvisor.com/Restaurant_Review-g187870-d716268-Reviews-L_Incontro-Venice_Veneto.html) In general, avoid food in the main tourist drags and any menu with pictures of food on it, lol.
We stayed here (http://www.tripadvisor.com/Hotel_Review-g187870-d646507-Reviews-Oceano_Mare_B_B-Venice_Veneto.html) and loved it. Tucked off the main drag in the Arsenale area. We were not after the super-swanky “honeymoon suite” experience, so it may not be everyone’s cup of tea because it is not fancy, but we thought it wascomfortable, homey, and lovely. The owner, Nicola, is awesome.
It will be hot. PPs have covered the issue of clothes, but definitely pack comfortable (but chic) shoes, a couple of light sundresses/maxis, a wrap or cardigan to cover your shoulders in churches, etc. A floppy hat and some sunscreen are also musts.
Try not to cram too many cities into your trip. There is the temptation to see absolutely everything, because the airfare is so expensive, but you’re short-changing yourself if you spend too much time traveling and not enough time soaking up each city’s personality!
Everyone has romantic ideas about renting a cute little Fiat in which to zip around twisty Italian roads, but just… don’t. No cars if you can help it, especially if you’re spending most of your time in the major cities. The train system is fast, cheap, and generally very reliable (just be on the lookout for train strike days, which are often posted in advance). We packed light and planned our itinerary so that our last hotel had a washing machine, which made train travel much easier. Resist the temptation to overpack! If you forget something, you can always buy it over there.
Also, a day trip to Pompeii is a great idea if you have time. I found it very fascinating and moving. One of the best experiences of my travels anywhere!