Oh God, I LOVE Italy, never had a bad time there, and would recommend pretty much everywhere I've been:
Rome, I'd say a minimum of 2 full days, I could have done 2 days on Roman remains alone, let alone the Vatican and wandering the city, and the churches, and, and, and...
Tuscany and Umbria are gorgeous - Florence, Siena, Montepulciano, San Gimignano (I've spelt that wrong, doubtless!), Perugia, Pisa, stunning landscapes and countryside too.
Bologna and Modena. OMG Bologna is one of my faves, it's not one of the big hitters, but it's still beautiful, and the food is the best in Italy. For a country with incredible food, that's truly saying something. We went to some kind of buffet restaurant on a Sunday in Modena, filled with Italians, and it was amazing. DH and I still talk about with the same admiration as meals at Peters and Gordon Ramsay, and was probably about £20-£30 a head.
The Amalfi coast is stunning, we stayed in Sorrento, but it was August, absolutely rammed and HOT. We had really wanted to explore the whole coast, but it was just too packed. But I can't wait to go back out of season, and aid really like to see some of Naples. Pompeii was out of this world, somewhere I'd wanted to visit my whole life, and didn't disappoint (and your volcano could be Vesuvius!). Now want to get to Herculaneum and the museum in Naples to see all the treasures.
I went to Venice for my 30th, when I saw St Mark's Square for the first time, I cried, it was so beautiful and iconic (I was also about 6 weeks pregnant, so hormones may have played a part 😂).
On my list: Milan, Sardinia, Sicily, Verona, the Lakes, the Cinque Terra, Lucca...I could happily visit Italy for every holiday.