Can’t speak to Greece, but I visited Italy for a couple weeks long ago
Rome: pizza is ass, but the sites are great. When I went the forum was free to visit, but now I think you have to buy a pass for the forum/collosium/Palpatine hill combo. Vatican museum is also a must see, just remember they have a dress code for the basilica (no shorts). If you use public transport, buy the tickets immediately. They will do random audits and the police will literally haul your ass to an atm to get money for the fine if you’re caught without one. Rome is pretty safe in the tourist areas, just don’t look like a dumb tourist and don’t keep your money in an obvious spot
Naples: Pompeii is a must see. Herculaneum too. Naples has some great museums. Good food also - best caprese sandwich I ever had was from some food stand a taxi driver recommended (I figured he was scamming us, but he was telling the truth lol). There is also the ferry to Capri, but unless you time it right you might be better off getting a place for the night there rather than try to make the last boat out
Florence: good food and museums. I forget when, but they had a “culture week” where admission to all the main museums was free. They also have a quirky museum dedicated to renaissance science and they supposedly have shit like Galileo’s finger in a jar, but it was closed when I visited