What's the ONE thing I need to do in Italy?

Wait. You’re going to Italy? Since when?

Go To The Calcio In Florence!!

Rome - take a private cooking class one night for dinner. eatingitalyfoodtours
I'd also recommend going to the Trevi Fountain and Spanish Steps area at dusk.

Venice - Take the vaporetti to Murano & Burano

Agree that a stroll around Rome at dusk or just after dark is a magical thing....walk from Navona, to the Pantheon, all the way to Trevi. Or stroll across the river and gaze at the Dome of St Peter's; all are beautifully lit. IIRC, you're traveling with a teen--you can turn an evening passegiata into a gelato crawl with just a little planning. Avoid the industrial gelato shops and make a little effort to seek out the artisanal stuff. I love to do this so much that I'd rather have a late afternoon snack, skip dinner, and go on a walking gelato binge.

Here's a sample walk after dark/sights/gelato itinerary:
--start out at Gelateria dei Gracchivia di S. Pantaleo, 61. Everyone in your party should get a small, with two different flavors, to maximize tasting potential, and then walk to piazza Navona. Walk slowly, eat gelato, enjoy the fountains lit up at night and the general silliness of Navona after dark: teens on school tours, elderly couples strolling, street performers, confused/jet lagged tourists, etc.

--while in Piazza Navona, go to Tre Scalini's takeout counter and get a "tartufo", which is a semi-famous chocolate ice cream truffle. One is enough to share; find a convenient bench in Navona and enjoy. Tre Scalini website (cause you love links): LINK

--stroll toward the Pantheon, along the via degli Straderari (look for the "book fountain" on the right, set low on a wall) until you get to the Caffe Sant'Eustachio. It's open until 2 am most nights; either have a cheap & quick stand up espresso at the bar to keep you going, or (more expensive) sit outside in the little piazza and have a coffee granita (con panna, with cream). Look at the facade of Sant'Eustachio church, with the stag...he's the patron saint of firefighters b/c he & family were roasted alive inside a metal bull (early martyrs of the Christian church).

--keep strolling toward the Pantheon, go around to the front and sit on the steps of the piazza della Rotunda. Enjoy the view of the facade--the doors will probably be closed, as it's not open all night....the structure is still the largest unreinforced concrete dome in the world. Those Romans were genius with concrete.
--if you're ready for more gelato, stand with your back to the Pantheon and follow the street dead ahead and to the left--via della Rosetta. Turn right at the next corner and you're directly in front of https://www.ilgelatodisancrispino.com/en/home-eng/, an establishment that kicked off the artisanal gelato movement in Rome more than a decade ago. My favorite flavors from San Crispino are the meringue flavors: lighter than gelato, more of a semi-freddo with crispy meringue bits. Or the fig & walnut.....

--now head toward the Trevi fountain. You can follow the narrow, tourist choked via dei Pastini, or you can go a block deeper and follow via delle Colonelle to piazza Colonna to via dei Sabini, emerging at the side of Trevi. It's pretty at night, and it is usually less crowded.

--from Trevi, turn toward the river and head to Piazza Venezia....where the ugly-during-the-day monument to Vittorio Emmanuele (aka the white marble typewriter) is much more attractively lit at night. If you can hold more gelato, Gelateria Aracoeli is close by...

--from piazza Vittorio Emmanuele, walk back in the general direction of Navona via the Corso Vittorio Emmanuele, passing in front of the Scholar's Lounge (Guinness, because gelato alone is not a balanced diet) and Il Gesu, the mother church of the Jesuits. Il Gesu is noted for being the first baroque style church in Rome, as well as a relief on the facade depicting a Jesuit with his foot on the neck of a Protestant. If it is still open, go inside and marvel at the over the top baroque interior...so much lavish decoration, you won't know where to look first.

--Still need a gelato nightcap? Keep going up the Corso until you hit Largo Argentina...the large hole in the ground (excavation) functioning as a stray cat sanctuary. Look down into the hole on the via Arenula side, near the mid point, and you'll be gazing at the actual spot where Julius Caesar was assassinated (the curia of Pompey's theatre). Go about a block up via Arenula to Gelateria Artiginiale Corona Roma, a sister shop of the aforementioned Aracoeli.

--continue up via Arenula toward the Tiber, you can look at the river by night & head into Trastevere for more fun...or walk a little bit downriver to cross the Ponte Fabricio, the oldest bridge in Rome (built in 62 BC) over to Isola Tiburina.

Sigh. Now I'm going to need a gelato to get through the afternoon.

Florence was by far my favorite of the three.

Lots of places to go but try not to miss the Chapel of the Princes, Mausoleum of the Medici Family. Very different style (baroque) than almost anything else you will see there and pretty amazing.

I also really liked Santa Croce church as well as I like history and it contains the tombs of Michelangelo, Rossini, Machiavelli, and Galileo. Also has art by some pretty famous artists like Donatello.

quote:

---

Well I wouldn't say this is the "one" thing, but for something completely different, check out Santa Maria della Concezione dei Cappuccini, aka "The Bone Church". It's a quick visit and unique.

---

I wandered into SM della Concezione one day, bought a ticket, gawked at the bone art (and the odd mineral bone smell)...on my way out, I asked the desk attendant for a restroom, in Italian. She replied, deadpan, in English: No Bathroom. Only Bones.

quote:

---

Rome - take a private cooking class one night for dinner.

---

This. One of the highlights of our trip.

Day trips to Cinque Terre and Pisa were worth it if you can spare the time.

Wake early in Venice and explore at daybreak. You’ll have the city almost entirely to yourself until after breakfast.

I will second (or third or fourth) just spending time walking around. It’s amazing.

Gotta try the tacos