We flew BCN-FCO (this was in early June) and when we got to the airport, the lines to check-in were insane. We had arrived about 3 hours early because we had heard that check-in and security lines were going to be long due to the influx of travelers, but luckily, our airline (Alitalia) had virtually no line and we were through security in less than 30 minutes.
It was hard to find information regarding what museums/sites you should book in advance to avoid not only lines but also not being able to procure tickets, so I'll list them out for anyone who is thinking about visiting Italy:
Vatican Museum
Colosseum (if you want to visit the underground)
Uffizi Gallery
Accademia Gallery
Cenacolo Vinciano (da Vinci's The Last Supper)
The day after arriving in Rome, we headed to the Colosseum, Roman Forum, and Palatine Hill. We arrived at the east entrance of the forum about three hours prior to our Colosseum entrance time and slowly made our way through the forum and hill before going to the Colosseum.
The following day was spent in Vatican City going to the Vatican Museum and visiting Saint Peter's Basilica. The line to get in to the basilica looked line a Disneyland ride line, but it went quickly so don't be deterred if you visit.
The rest of the time in Rome was spent at a few more museums, tourist spots, and just wandering around eating gelato. So much gelato.
Borghese Gallery
Spanish Steps
Trevi Fountain, which surprised me due to its location. I had always imagined that it was in some wide open piazza, but it's instead boxed in between buildings on each side.
Victor Emmanuel II Monument
After 5 days in Rome, we took the train to Florence.
View of Florence from Piazzale Michelangelo
Ponte Vecchio, which I thought would contain apartments or something along those lines, but nope, just jewelry stores.
Uffizi Gallery
Botticelli's The Birth of Venus
Accademia Gallery
Cathedral of Santa Maria del Fiore
We booked the earliest possible time slot to walk up to the top of the duomo. Highly recommended during the summer before it got too hot in the afternoon. This is the view from the top.
And then right after, we climbed the bell tower.
Travel tip: assuming you have the requisite tickets, instead of standing in the line that wraps around the piazza to get into the cathedral, go in through the side meant for people to see the Santa Reparata. You get into the cathedral and are supposed to go down to essentially the basement, but you can just stay on the cathedral floor and there's no line.
Next we took the train to Venice. While beautiful, Venice was exactly what I had pictured and I can see why a lot of people make it a day trip. The island is so small that you can easily walk across it in 30 minutes and you can see all the "must see" sites in a day.
Rialto Bridge, which like Ponte Vecchio in Florence, is lined with stores.
Doge's Palace
Bridge of Sighs, seen from inside Doge's Palace (and you can walk across it from inside Doge's Palace as well).
Scalzi Bridge
Saint Mark's Campanile
View from the top of Saint Mark's Campanile. Unlike the bell tower in Florence, this one has an elevator that takes you to the top.
Saint Mark's Basilica
Ponte delle Guglie
Rialto Bridge at night
Saint Mark's Square
We did manage to make a day trip to Murano. Burano was out of the question because my travel partner got sea sick from the short 15 minute vaporetto ride to Murano; the 45 minute ride to Burano would have been worse. If you plan to buy glass work on Murano, be sure to walk well into the island as the first few shops closest to the first vaporetto stop on the southern edge of the island seem sell the same items for 2-3x as much as the shops more towards the center of the island.
Our final stop in Italy was Milan. Once again, we took the train. The convenience of European trains cannot be overstated. The tourist areas in Milan are pretty well concentrated in the center and all within walking distance from one another. We opted to stay near the central train station (scroll down to find out why) and getting to the center only required a <10 minute metro ride.
Cenacolo Vinciano
The Last Supper. You get 15 minutes to view the painting in a group of about 30 people before you need to move on for the next group to come in.
Milan Cathedral
View from the top of the cathedral
Inside the cathedral
Galleria Vittorio Emanuele II, basically a fancy mall
Milan Cathedral at night
From Milan, we took a day trip to Cinque Terre. It's about a 3 hour train ride from Milan; we took the train from Milan to Monterosso early in the morning (thus our decision to stay about a 5 minute walk from the central station) and from La Spezia to Milan at night. Be sure to buy the Cinque Terre Card as it gives you access to the trains running between each town, the hiking trails between the three northern towns, and the restrooms at each train station.
Vernazza
Manarola
Riomaggiore
Up next, the remaining countries from my trip abroad.