Seriously, I was just curious about this. I mean I can damn near identify a certain food item to be particular to various regions/culters, but I can’t for the life of me think of anything that would be indicative of an Italian breakfast.
To generalise wildly: Adults: coffee (espresso, cappucino or caffelatte) and a cornetto (sweet croissant), or a doughnut - at a bar, often standing up. Children traditionally have cookies dipped in hot milk. But plenty of people just eat cereal.
Just one?
Give it to me. 
I had an Italian roomie for about a year. Every day, his breakfast would be a glass of cold milk with cappucino flavoured biscuits/cookies.
I can mostly confirm this; I’ll just add that the use of cereals took off only in relatively recent times. Adults often get their breakfast cappuccino and cornetto at a bar to socialize (football, TV and politics being the hottest topics), even if the official excuse is “because espresso coffee from a bar tastes better”.
Then of course there are lunch times and some additional coffee breaks, which is another occasion to socialize in the friendly environment provided by the bar. Ok, sorry, that’s out of the original topic.
Sunday breakfasts at my Nana & Papa’s house consisted of coffee and cookies/pastry as well. When we were kids, we got our coffee with tons of milk and sugar in it; grown-ups drank it strong and black. Among the usual selection of munchies were biscotti, anisette cookies, almond crescents, toffee chip cookies and pizelles. Breakfast would gradually segue into lunch - when the fruit and cheeses came out, we knew breakfast was done.