which tickets must be purchased in advance to avoid very long lines?

The Uffizi, Accademia, Colosseum, and Vatican Museums all should be booked in advance. If you wait until you're close enough to know the weather you likely won't be able to get tickets (or if tickets are available they won't be at ideal times).

The Uffizi, Accademia, Colosseum, and Vatican Museums all should be
booked in advance. If you wait until you're close enough to know the
weather you likely won't be able to get tickets (or if tickets are
available they won't be at ideal times).

This is true even in the Spring?

Thanks.

Yes, even in the spring.
I wouldn’t worry about the Medici Chapels, Duomo Museum, St.Croce, or the Barbagallo. No lines in August.
The Pitti Palace neither. When we were there in late August, we walked by everyday, no lines. EXCEPT for the free entrance on the first Sunday of the month (we left September 2). That was the only day we saw a line.

If there is something I really want to see and would be upset if I missed it, I prebook, always, like the David. If it is something I can miss, like climbing the dome, I don’t stress and worry about lines. If I missed it, I miss it,

Also, keep in mind Rick’s advice about going later in the day. When we were in Florence we had Accademia tickets for 7pm. We passed by at 5:30 and there was no line. We asked and they let us in early. Now, if we had no reservations I don’t know if they would have been so accommodating. Also, very short line when we left an hour later.

Just back from Rome and I can assure you there were crowds at the Vatican and Colosseum even in February. You definitely want tickets in advance.

At this point the regular entry tickets for late March / early April are probably all sold out. However, you may be able to still purchase a tour either from the site itself or from another company that will get you in. There’s some risk that other companies won’t actually have tickets and will cancel your tour the day before, but the alternative is either wait in the interminable line or try and buy a tour when you get there from a company that has tickets. We were there in January and booked an audio guide tour of the Colosseum and Forum with C.I.S. Tours through Get Your Guide. It was just entry tickets and an audio guide app that we didn’t download, but the CIS rep was there on schedule and we got in with no issues. Good luck!

For Florence, a good way to gauge ticket availability is to keep checking the official ticketing site:
https://www.b-ticket.com/b-Ticket/uffizi/?_ga=2.226285597.605480266.1741457147-1356136015.1736693461
Looking at the Uffizi Gallery for end of March/beginning of April, I just scanned but didn't see any time slots that were sold out. For the Galleria del Accademia there are several time slots sold out, especially in the mid morning, but plenty still available.

You could check on a regular basis and if you find they are filling up then you know it's time to reserve.

Good luck.

We were at the Vatican in May 2022 and because I waited until about 10 days before our arrival, I had to purchase Vatican tickets via a 3rd party - pretty sure it was The Tour Guy. Purchase and ticket pick up went great; just paid a few euros more than purchasing directly from the Vatican website.

We went to the Coliseum in 2023 so I just set an alert and purchased tickets via the official website when they became available.

As another poster stated, check the official sites and when times are booked up; it is time to purchase tickets. If the sites you want to see are booked on the days you want/need; you may want to consider some “reputable” resellers. Just do some research about the reseller before purchasing.