Italy Itinerary for 25 nights

Twenty-five days is a wondeful stay, but I'd move less (and therefore spend less time dealing with hotels and other logistics).
I'd do Orvieto as a day trip from Rome; it's less than an hour, and I'd drop either Sorrento or CT, choosing one coastal location. You might think of whether you'd most enjoy hiking in CT or the range of activities available near Sorrento. I'd stay in Florence for 5 nights and do a day trip to Sienna; Florence offers a much wider range of options than Sienna. And stop in Assisi for a couple of nights on the way from Florence to Rome. Assisi is wonderful, but you're wise to plan to spend a couple of nights.

Not a bad plan, but you are moving a LOT. Combine Florence and Siena: Stay in Siena and take a bus to Florence one day... or two. Rent a car when you are done busing to Florence and see some hill towns. Keep your car for Umbria and see Assisi and Orvieto. You should, IMO, stay in Assisi and day trip to Orvieto with said car. You can then also see Perugia, Spoleto, Spello, etc., in Umbria. Yes it's a long train day Venice to the CT, but we liked it our first trip to Italy.
Do make reservations in advance so you aren't spending precious trp time looking for a place to lay your head.

I would do this. Fly in Naples. Sorrento 5 nights Rome 5 nights Orvieto 3 nights (day trip to Assisi) (with car) Siena 3 nights (with car) Cinque Terre 3 night Florence 3 nights
Venice 3 nights Fly out Venice Total 25 nights.

I respectfully disagree with Rick in Bellevue. Fly into Venice and out of Rome. Venice morning flights are notoriously early and the airport is a pain to get to early.

All I wanted to say is don't drop Sorrento as Sherry suggested.. It's absolutely wonderful and our favorite place in Italy. Not only is the town charming, there's also so many fun options of places to visit while you're there if you wish.

I think your itinerary is a good one. It's busy but not overly busy, and transit times are reasonable. The only change I'd suggest if using public transit is to visit Siena before Florence, as it's easier to get to Assisi from Florence. Venice to Cinque Terre is a fairly long train trip (about 6 hours), but you've planned 3 nights and 2 full days there so not an issue. You'll be able to explore much more in Tuscany/Umbria if you have a car. If you do decide to rent a car, keep your original order, rent it when you leave Florence and drop it off in Orvieto (use it to visit Civita before dropping it off). If you plan to visit Paestum, you could keep the car and stop on the way to Sorrento (slight detour) and then drop it off upon arrival in Sorrento. Enjoy your trip.

It looks perfectly fine to me, with the caveat that I haven't traveled to Assisi or Orvieto, so I don't know how the connections are and what order to visit them. I like the idea of starting in Venice. If you can easily get a flight home from Naples, then you could go from Orvieto to Rome and end in Sorrento, which saves a little backtracking to Rome. You said "last minute" which makes me think that you may be going too late in the year to risk the Cinque Terre, unless you have a back-up plan. If you get grey skies, it won't be very pretty, and if it rains, there will be little to do except count the puddles. Late September and even early October should be okay. Even so, try to get reservations that you can cancel 2 or 3 days in advance, in case there is bad weather on the horizon. Between Tuscany and Umbria, I'm sure you'll find more sights to see. You go through Florence to the CT. But you want to be there early, so you pay for it with a couple hours of backtracking. Not terrible. Just a little inconvenient.

Laurel's right about flying out very early in Italy. They'll just be sending you to another European gateway city where you'll sit the rest of the morning before flying home mid day. Rome flights directly to the U.S. leave late a.m. That way, you can get a good night's rest and get up at a normal time. Anyone flying out @ 6:00 a.m. will have very little sleep, and returning flights to the U.S. are long enough.
Go from north to south.

Agree it is better to fly into Venice and out of Rome. We did a similar trip and considered flying out of Naples but Rome was more convenient. We stay near the Termini and can have breakfast and walk to the station at a reasonable hour. While it seems you have a lot of stops we often travel that way, although we prefer 3 nights in each town, 2 nights work fine when the travel time is short.

With the exception of the CT, every time that we have stayed only two nights in a place, we have regretted not staying three. That was true of Siena, Assisi and Orvieto. When you do only two nights, you only have one full day in that location - not enough to enjoy the slower pace of Italy, imho. I would steal a day each from CT, Sorrento and Rome and add them to Siena, Assisi and Orvieto.

I wonder who invented this idea that 2 nights = 1 day? If one arrives in Florence at 9am, deposits his luggage at the hotel and goes sightseeing, he doesn't lose a full day. 2 nights can be equal to 2 full days in my book.

Sean, by "full day" I mean one unbroken day, from waking up to going to bed. As for your scenario, I don't know many people who will want to plan their travel to arrive in a city by 9 a.m.

It's worth changing one's sleeping patterns on a trip ;)

Sean - I am trying to follow your math to get 2 whole days. I do not think I have ever arrived at a destination in Europe at 9 AM, but regardless, that is the first day and you would have to be getting to your place to stay, checking in, quick orientation, and then out and about. In the last 12 years of going to Europe each summer for a month, I have never hit the streets before noon on any trip.

Let's say you leave Rome at 7:30 and arrive in Florence at 9:00. By 10-11 am you'd be already exploring the city. Obviously this doesn't work for sleepyheads, but it works perfectly for early birds like me.

As long as it works for you Sean. That is all that is important. For others maybe not so good.

How much time it takes to change locations varies. It takes a long time to get to the train station in Venice because the vaporetto is sloooow. It takes a long time to get to the CT and Sorrento because they are relatively remote and only served by the sloooow regional trains. In and out of Florence is quick, you're often only 5-10 minutes from the train station. If you avoid rush hour, it's a 5-10 minute taxi ride to your hotel in Rome. I maximize my sightseeing time by using evening trains after most attractions are closed and I'm tired anyway. Eating out isn't a priority for me, so I have picnic dinners on the train. Saves money too.

Chani, that's also a solution, except you wouldn't be able to go back to your hotel to shower and relax after a day of sightseeing. Check out is at 10:30 in some hotels.

You are going to have a great time. I like your plan. I went for 22 days from mid-May to early June. Know that the drive (and I'd guess train too) from Venice to Cinque Terre is long! It also takes forever to get in/out of Venice and into/out of your specific town in Cinque takes a long time too. Expect 6 full hours to switch from these two towns. Is this the best order? I'd advise skipping Sorrento too. First, add a day, maybe two, extra in Florence for Pisa could be a way to distribute the Sorrento days. Yes, Florence has to be 3 full days to take advantage of the Florenze card - a total must for museum goers. Then a day trip to Pisa is fun! Six days in Rome gives you the inner city. Perhaps a day or two extra in Rome allows for sights on the edges of town and gives you flexibility to catch an audience with the Pope (book early and free with Saint Susanna/US Catholic church in Rome - pick up ticket in person Tuesday evening) and allows you to hit the Vatican Museum on a lower traffic day (make a pre-paid reservation for the Vatican).
My trip was very similar to your plan. In reflection the top hits are Rome, Florence, and Venice, in that order. The other towns are gravy but don't skip on the real meat of the country.

We flew into Venice and out of Rome in May and that worked just fine for us. Venice was also a fun romantic city to start the trip with. We then took the train to Cinque Terre and it was the longest travel day went down to Florence on the high speed and then transferred again in Pisa to Momterosso (where we stayed). Took six hours total but worth it. We stayed in Florence for a few nights and then moved to Pienza in Tuscany, which was great because it was central to the region and we took day trips by car to the various hill towns. For Rome. It's going to feel pretty busy after Tuscany we regretted even having a full three days there, but everyone has their own preferences of what to do. I was glad to go once, but I'd prefer time in Tuscany or Cinque Terre. It might be worth reading the guidebook to determine what you want to see and then decide how long to stay in each place?