I have.a number of attraction tickets already lined up during our 16 nights in Italy - mid to late September. Some of the days already are starting to look a bit full, but I imagine that many of the stops will be just an hour or less.

I don’t want to overplan and clog up the days with just hurrying around from one reservation to the next, but also want to be smart about not wasting time in long lines.

What would be your (up to 5-6?) must-reserve attractions in each city?

They all vary from person to person, obviously. If you like art, obviously some of the major museums might be necessity.

But for me, and for my interests, I book advance:

Venice: Basilica San Marco + museum, Doge's Palace - specifically a guided tour. I have also booked the Museo Correr in the past (reservation may not be required) and the bell tower. Also booked Accademia on my last trip in 2022, but like Correr, was not busy.

Rome: Vatican Museum, Colosseum. Also made a point to arrive at St. Peters at 7am (no reservation needed) to avoid lines

Florence: Accademia, Uffizi. I went to Florence in 2006 well before it was super popular and reservations were needed back then (and I had to call!). Nowadays, it may be necessary to do advance tickets if one wants to go up to the dome of the Duomo. Not sure about other museums and landmarks.

You won't have time to see everything, so don't even try... Prioritize, prioritize, prioritize. September is still very much "high season."

I haven't been to Venice since well before the pandemic, and I'm not sure what sights would require reservations these days. Doubtless the Basilica, probably Doge's Palace and the Accademia, perhaps the Campanile. We really enjoyed the Peggy Guggenheim house, a couple of other palazzi that overlook the Grand Canal, and trips to the islands. It's a great city for wandering/exploring.

Florence planning can be tricky if your days there include Saturday/Sunday when some main sights aren't open (depending on the weekend). You need to check open days/hours and organize your sightseeing by the day of the week. The city is small and very walkable, so geographic distances aren't as much of a determining factor. Things where I would want reservations: Uffizi, Bargello, Medici Chapels, Accademia, Duomo and climbing to the lantern, Santa Croce Basilica, Palazzo Vecchio, Brancacci Chapel. And there are many, many other wonderful things to see that may or may not require reservations... the Baptistery, the Opificio delle Pietre Dure, Palazzo Medici Ricardo, the Great Synagogue, stunning churches and their art.

Rome is in an altogether different category than Venice and Florence in both size and the number of amazing sights that are not quick stops. You need to be a bit ruthless in prioritizing your sightseeing. You can make reservations for the usual stuff: Vatican Museums, St. Peter's and climbing to the top of the dome, the Scavi tour under St. Peter's, Borghese Galleria, Coliseum, Forum, Pantheon. Other things we enjoyed where reservations may or may not be required: Capitoline Museum, Massimo Terme Museum, Doria Pamphilj Galleria, Domus Aurea, Ara Pacis, the Castello. You might want to peruse the list at this website:

https://www.walksinsiderome.com/blog/rome-tickets/

Don't get caught by the travel guide "you must list". You will forget all of them long before you forget watching the sun rise over a canal in Venice, any canal. Enjoy the little things in life and the big queues vaporise.

There is already an unhealthy list of places that require reservations, iff you want to see them. What are your personal musts?

if it would interest you, the Domus Aurea excavation of Nero’s buried palace in Rome needs reservations as there are only a few tours per week.

I've never reserved anything in Venice. We waited in line to get in to St. Mark's Basilica. (There were three of us, so one stayed in line while the other two meandered around the piazza.) The only time I went up the Campanile, there was no line, but that was a long time ago. Likewise the Scuola Grande di San Rocco and the Basilica Santa Maria Gloriosa dei Frari. We visited the Ca' Rezzonico, which I highly recommend, without a reservation.

In Florence there are no reservations for the Duomo, and no tickets. There is almost always a long line to get in; I would suggest just admiring it from outside; a lot of the art that used to be inside is now in the museum of the Duomo. You can reserve time slots for everything else associated with the Duomo. There are three passes: the least expensive, the Ghiberti pass, is for the Museum of the Duomo (spectacular), the Baptistery, and the remains of the ancient Church of Santa Reparata (under the Duomo). The Giotto pass adds the Belltower to that, and the Brunelleschi pass also adds the Dome. The Brunelleschi pass sells out, so if you want to climb the Dome, you should buy that in advance on their website. At the moment, the first availability is 12 days from now. (Be aware that it's not an easy climb.) At the moment the Baptistery's splendid golden mosaics are being cleaned and restored, so you can't see the ones on the inside of the dome; this work is expected to take a while. Apart from the Brunelleschi pass, the others can usually be reserved the day before or on the spot. I've always bought them on the spot. (I climbed to the top of the dome back when there were no reservations, and there was no line.) Reservations can be made here:
https://tickets.duomo.firenze.it/en/...ugroup_id=3006

Although you have to pick a time slot, the passes are good for three days, which doesn't make a lot of sense to me. Having a reservation doesn't mean you won't have to wait in line.

You definitely need to make reservations for the Uffizi and the Accademia if you don't want to wait for hours to get in. You can optionally make a reservation for San Marco, but I don't know how much of a wait there would be if you didn't. (I was last there over 10 years ago.) I've never made a reservation for the Bargello, and I've been there recently. The Brancacci Chapel usually has openings at the last minute.

In Rome, the only things that really need to be reserved are the Vatican Museums, the Colosseum, and the Borghese Gallery. There are dozens of wonderful archaeological sites and museums in Rome that are never crowded and don't need reservations.

I make it a rule not to reserve more than one thing (or at most two) each day. You never know in advance how you'll be feeling about visiting museums on a particular day. I especially advise you not to rely on anyone's "must-see" list. If you never go to an art museum at home and don't know the difference between mannerism and expressionism, maybe you're not going to have much fun in an art museum in Italy, especially if it's crowded and hot. You're paying a lot of money for your trip: make it yours. If someone asks you later if you saw the Vatican, say "yes", even if you just looked at it standing in St. Peter's Square.

I suggest getting a good guide book of Italy with lots of photos. The DK Eyewitness guide to Italy is a good one. Browse through it and pick a few things you'd really enjoy in each city. Then spend the rest of your time meandering around, admiring fountains, relaxing in parks, and people watching.

The tour of the Scavi (excavations) under St. Peter's Basilica requires reservations which usually must be made several weeks in advance.

Excavations Office

Domus Aurea is only open a couple of days a week and entries are timed. If this is something you want to see, you should make a reservation.

https://colosseo.it/en/tickets/domus-aurea/

Seriously consider doing the Pristine Sistine Early Entrance Small Group Vatican Tour. We did it and did not regret spending the money at all. I initially hesitated spending the money but after speaking to someone who went early and was squashed into the Sistine Chapel with a load of people, I decided it would be worth the price. We were able to move about freely, to take it all in and we were not rushed. You will go in very early. When we came out and saw the extremely long line to get in, we were happy to have splurged. BTW there are other companies that do skip the line yours of the Sistine. A friend recommended Walks of Italy so we went with them. My regret with Rome was not getting advance tickets for Galleria Borghese. If it is something that interests you get tickets in advance.