Many factors to consider. Visiting major popular destinations like Florence, Rome, Venice, Amalfi Coast, etc. or off the beaten path? Rental car or no rental car? Peak season (April to October) or low season (Nov to March)?
If you visit major destinations like Florence, Rome, etc. in high season you should expect to spend a minimum of €150 to €200 a night for a hotel in the city center. I am talking about a nice 3 star hotel or even a 4 star with breakfast included. Nothing super luxurious but nice. Less hyper popular destinations may be about 50€ less. In low season prices drop significantly.
Prices of restaurants are the same year around. In Florence, Rome, etc. you can have a nice abundant meal at a sit down restaurant for about €40 p.p., including wine. Off the beaten path prices may drop to under €30 pp. once again nothing fancy, but nice sit down restaurants.
If you stay at a hotel that offers breakfast (most do offer buffet type continental breakfast), you can easily skip lunch or just have a cheap quick snack on the go (panini, pizza, etc.) for under 10€.
If you plan on visiting sights I recommend not to go to sit down restaurants at lunch, as lunch will take 2-2.5 hours of your time at the very minimum. Restaurants in Italy have slow service as they are very understaffed by US standards, both in the kitchen and on the floor. My American relatives, some of whom waited tables in youth, told me they would quit after an hour if they had to work all the tables an average waiter works in Italy. Waiters in Italy have to serve an insane amount of tables and they can’t possibly provide fast service in those stressful work conditions (ironically as hard as they work, they don’t even get a penny as tip, at least not from Italians). Therefore leave your formal sit down restaurants for the evening dinner, when you are not in a rush to head to the next museum.
Museums are not expensive, and rarely cost more than 15€ each.
Transportation within the city is cheap if you rely on your feet and the occasional buses. Taxis are expensive, but in most cases you will need a taxi only when you arrive and depart (from/to the airport or train station).
If you rent a car, plan on $60 per day for the rental including full no deductible insurance. Gas is expensive (about 2€ per liter at self service pump), but distances will be short and cars are fuel efficient, therefore fuel won’t be more than 10-12€ per day at the most as I doubt you will log more than 100km a day on average.
If you travel by train between cities, that cost will vary, so you need to calculate the cost yourself based on your itinerary.
Shopping and souvenirs is entirely too subjective.