3 week itinerary in Sept. - too busy?

It's obvious that you've done a lot of planning, however you may have packed too much into your itinerary. After you fly into Italy, you need to take it easy for a day or two giving yourself time to recuperate from the flight. Other than the Vatican Museum and The Borghese where you need reservations, just roll with the flow--not planning so much.
When you get to Rome, the best thing you can do is get familiar with the mass transit system, especially the buses. They're fast and efficient and can get you all over town in just a few minutes. You'll need to save your legs.
After you leave Rome, I would suggest going straight to Florence and taking in the sights there. Then get your rental car as you're leaving town. We stayed in an agriturismo 4 nights--touring Tuscan towns like Certaldo, San Gimignano and Volterra.

Sienna is best done as a day trip from Florence via bus, as it's just an hour's ride down there. It's very congested and parking is difficult.
Pisa, other than the tower and their cathedral, is not that interesting and many travelers bypass it. Lucca is a very quiet place and I would rather spend my time elsewhere.
I've never been to Ravenna, so I cannot help you there. I would go straight to Venice from Florence and skip Ravenna. I'd rather spend my time in a rental car in the Dolomites or the in country north of Venice. Or, take in Verona.
We're leaving in 3 weeks for Rome, and looking to take in the sights we missed last trip. It's a place everyone goes back as there are so many things to see. We fly out of Venice--a place I've been to about 10 times.

I think your plan looks fine, I would put some specific churches into Roma based on your interests: San Luigi dei Francesi (near Piazza Navona but I think it has odd opening/closing hours. Santa Prassede, near the Basilica of Santa Maria Maggiore; Santa Sabina, on the Aventine Hill, near a garden with great views of the city.

Do not skip Ravenna, you will love it. I also disagree that Siena is best as a daytrip - it's much better after the daytrippers leave. In fact, although daytrips are often necessary, I can't think of anyplace that is better as a daytrip rather than a couple of nights' stay.

I think your plan looks lovely. Because you are not always changing places, you can adjust your itinerary if you see more or less than you had planned on any particular day. This is what one of my itineraries would look like. I don't always stick tightly to my itinerary, but I prioritize the sights and go from there...incase there is something not on my list I decide to do or see. Have fun!

I want to echo the advice not to skip Ravenna. With all due respect, I believe that only someone who hasn't been there could advise you to skip it. Also, don't waste your time there going to a beach. Ravenna has some of the most astonishingly wonderful mosaics from the 5th and 6th centuries anywhere in Europe. You should plan to visit, at the very least the church of San Vitale, the Museo Nazionale, and the Tomb of Galla Placidia (all in one locale), and the church of Sant' Apollinare in Clase. The town, itself, is lovely, with a pedestrianized center, and not too overrun with tourists, considering what a jewel it is.
I'll add that I don't think you've given yourself enough time for Venice, if, indeed, you expect to include a visit to any of the lagoon Islands.

Looks like a great plan to me and I do not think too rushed. I think you have ample time to see everything you want.

Can you return the car one day earlier and use public transit instead for your day trip to Pisa/Lucca ; those seem like places better to visit via public transit than car from Florence.
Alternatively keep the car and drive south this day and make a day trip to visit the towns of Montalcino, Montepulciano and Pienza ; towns mostly known for their wine but the surrounding countryside there is the best in all of Tuscany according to most.
Personally I would dedicate a day for this even f you still want to do Pisa/Lucca on one day and lose a day of seeing Florence sites.
Thinking beach near Ravenna in late Sept is likely not worth bothering with. Rimini is probably not far but don't think it would be the same there as a month prior.

thank you to everyone so far.. I guess I should have mentioned first that I've been to Italy once before, 16 years ago, a backpacker trip that started in Ireland that finished in Rome. Italy on that trip consisted of coming from Chamonix, France and arriving at Cinque Terre at night, doing the hike the next day. Off to Siena the following morning, still in Siena the next. Day trip to Florence the following day (I know, I know, be gentle.. ;) and on Easter Sunday, to boot). Train to Rome the next morning, 2 full days in Rome, then fly home the next morning.

@David - I have to agree with the others that Siena isn't a day trip, from my own experience, but I fully respect everyone's difference in taste. And I agree that Siena isn't car friendly, that's why we aren't renting the car till we hit Florence. My wife wouldn't be able to handle the bustle of Rome to Florence without a bit of a slowdown, another reason for Siena at that time. As for buses/metro in Rome, I've got a great app for the Metro, and we've mapped out the #117 that stops just outside the apartment and stretches up to Pza.d.Popolo... Thanks!

@Zoe - we do have some churches marked in Rome, we figure that they would fit somewhere in the later hours of Sept.4-5. Thanks for the names of specific ones to make sure.

@Rosalyn - we definitely plan to hit the all the sights you mentioned in Ravenna that Rick mentions in his book, we'll have lots of time as we slow down a bit again after the big Florence blitz. As for the beach, it was a passing thought as I saw how close it is from Google Maps, and my wife does have a kind of "sand from every beach" collection going. Also, I think I mentioned for the Venice days that we do intend to get to Murano for one of the lagoon islands.

@mreynolds - re: not having the car for Pisa/Lucca, it's a toss up and we could travel either way, whatever works out the best for convenience, $$, how much of the day is spent waiting for the buses.. but we won't give up the destinations. Going to other Tuscany towns on Sun.Sept 11 instead of non-Firenze card sights in Florence is a good idea - this is the most flexible of dates and $$ efficient for keeping the rental car days together. In fact, now that I see some photos, I think that's going to be a change to the itinerary! Thank you!

For the lagoon islands, we found Burano to be a little more charming than Murano and fun with all the colorful houses. It does take longer to get there as it is further out. I have heard good things about the church in Torcello as well, but sadly have not been able to make it yet.

Torcello is wonderful, the church is popular for weddings. Torcello was the original lagoon settlement, before Venezia was established.

Sun. Sept 4 - Vatican Museum and St.Peter's

First issue: St Peter's will be open but the Vatican Museums will be closed. They're only open on last Sundays of the month for limited hours:

http://mv.vatican.va/3_EN/pages/z-Info/MV_Info_Orari.html

Mon. Sept 5 - Ostia Antica….

Second issue; Ostia Antica is closed on Mondays:

http://www.ostiaantica.beniculturali.it/en/orari-ostia-antica.php

I would switch your Sunday and Monday itineraries?

Jeez, how did I miss those.. I even had it in my notes that the Vatican Museum is closed on Sundays! And I guess I assumed the Ostia ruins were open daily like the Forum ruins, didn't check the book to confirm.. Well, that's why I ask the questions. The help here is fantastic. Thank you Kathy, that's an easy switch.

I think it looks good. As long as you don't mind the short 2 days stops go for it. I did something similar over 17 days and loved it. Got to see a lot and plan longer trips to certain cities the following return trips.

DK, Sunday, Sept 4th - which you are now planning for Ostia - is a first Sunday of the month when state museums are free. Ostia Antica is one of those (see that note on the website). Free Sundays are absolute madhouses at 'most popular' places like the Uffizi, Colosseum, Accademia, etc. but I'm guessing that Ostia, while it may be busier than normal, will not be completely overrun as it doesn't appear on most must-see lists.

Just thought it worth the mention. :O)

It was in 1989, but one of my most memorable travel days ever was my day trip from Florence to Lucca and Pisa, in that order. By doing it this way, I got to Pisa in the late afternoon; the souvenir vendors were literally packing up, and the marble in the Field of Miracles looked lovely in the afternoon light. For Lucca, I rented a bike and rode around the walls - wonderful views of the city.

Buses and trains connecting the triangle of Florence, Pisa, and Lucca are all frequent, so there's not much waiting, and they're cheap too.