Well you're in luck because the Trevi Fountain's unveiling/re-opening is scheduled for November 3rd, however the Spanish Steps are closed for a spruce up. You can't walk up/down them at the moment.

The Colosseum in November should be okay for a walk up if you see it late in the day, after 3.00. Otherwise book a skip the line ticket through the official site - Coopculture.

The Sistine Chapel is accessed by the Vatican Museums which is pretty much busy all year round so also pre-book a skip the line ticket.

St Peter's is free but there is a security line. It can get massively long so try and get there early.

Book your rail through Trenitalia.

Rome & Florence in 3 1/2 days
 

I'm leaving next week.
Besides the fast train to go from Rome to Florence, what else do I have to book before I leave??
And what train company should I google to book?

In Rome, I want to see:
Colosseum
Sistine Chapel
St. Peter's
The Pantheon
Spanish Steps
Shop on Via del Corso?
See Trevi Fountain although under construction
Trastevere?

In Florence:
Duomo
Piazzale Michelangelo - Statue of David
The Palazzo Vecchio
The Ponte Vecchio
Leather Market

How should I split these 3 1/2 days? Feel free to give me your opinions and plan my itinerary. We first fly into Rome late Thursday night and fly out of Florence on Monday at 6:00 PM.

Unfortunately Florence on Monday is a bust, since many major sights are closed. I hope you will be in Florence at least for all of Sunday -and will have reservations for both the Uffii and the Accademia. In Rome you will need reservations, preferably for first thing in the AM - to see the Vatican Museum/Sistine chapel.

I think your itinerary is quite ambitious for 3.5 days. You are missing so much of Florence that is worth seeing. The statue of David at the Piazzale Michelangelo is a copy and doesn't hold a candle to the real thing in the Accademia. Be sure to see that and to make reservations in advance.

I would recommend so much more but you simply don't have time. For starters, you could skip the Palazzo Vecchio and add the Uffizi.

Check out Walks of Italy and use them to see most of what you want to do in Florence plus you will get a great understanding of the history. We also used them for most of the things you want to do in Rome also. We don't normally do tours, but our time was also very limited and this made it easier to organize. The tours are limited in number and they use headphones so you can hear the information without having to be right in front of the guide.

When you have very little time, taking a tour risks spending at least part of your limited time seeing things that don't interest you, unless you use a private guide who will tailor the tour to your interests.

There's no real need to book your rail tickets in advance. The best discounted fares are probably already sold out, and I don't buy discounted tickets that have only small savings. The full price tickets are much more flexible, and buying tickets at the last minute is the most flexible option of all. Some discounts are available even at the last minute, by the way.

Florence on Monday is not a bust - yes, the Uffizi, Accademia, and Pitti are closed but plenty of churches and museums are open (Palazzo Vecchio and the Duomo for example, both of which you mentioned wanting to see). Make sure to look at the websites for opening/closing hours and plan your limited time effectively based on what you want to do.

It was recommended to me just recently to take a Secret Passages of the Palazzo Vecchio tour (they seem to be about 1 1/4 hours) so you could look into that if it interests you.

Also the Palazzo Vecchio is open later than many churches (save parts of the Duomo) so make use of that extended time)

The Colosseum ticket includes the access to the Foro Romano so if you are interested in that as well, you'll want to budget some time for both.

With your limited time, I'd suggest nixing the Piazzale Michaelangelo, yes it's a nice view (we walked up there once and enjoyed a picnic lunch) but it's not the real David and you're going to be pretty tight on time.

It seems like you have more Rome-based things so staying there longer would make sense.

The door near the front of the Sistine Chapel leaves you near a gift shop in a little connection space between parts of the basilica and the apostolic palace so you'll actually be near the entrance of St. Peter's. If you go to through the Vatican Museums to get to the Sistine Chapel first then you could go right into St. Peter's. There's an audio guide below the steps, so if you want that, you'll have to walk down the main steps and look for the glass doors on the right (when facing the basilica facade).

Depending on what time you get into Rome on Thursday and how tired you are (don't know how long your flight is), you might want to take an evening stroll to check out the Trevi fountain, which is beautifully lit at night. You could get a gelato, see the Trevi, walk past the Pantheon which is near enough and take in a little of Rome at night. I would recommend going back to seeing the Pantheon from the inside since that was one of the things you specifically noted, but a nighttime stroll in Rome is always nice.

ducinaltum We have a 6:00PM flight from Madrid to Rome. I think it's a 2.5 hr. flight. So I'm guessing we will arrive in Rome at 8:30 PM on Thursday night. Then we have to get ourluggage and a cab to get to our hotel.

Also, if I'm only interested in seeing The David, why do I need to purchase tickets for the Accademia?

Because "David" is on display inside L'Accademia! Do not miss seeing this incredible work of art. The replica is not a substitute in any way.

you want tickets in advance because the lines can be tremendously long and you have no time to waste so you don't want to stand in line.

Oh good - then you won't be jet-lagged. I'd definitely recommend a late night stroll in that case, even just a little around your hotel to see Rome at night. Or you can get a glass of wine and sit at a restaurant/cafe near one of the big sights (like the Pantheon) where they have outdoor seating and the large lamps to keep the area heated at night. Enjoy!