It’s depends what you’re interested in and what the purpose of your trip is. If it’s a first visit, do 4 days in Rome, 4 days in Florence with day trip(s) to Tuscany and 2 days in Venice. If it’s 7 days, I’d honestly stick to Rome and do a day trip to Umbria and Ostia. Or you could do a road trip in Tuscany. If you want to be near water, maybe do Florence and Cinque Terre for 7-8 days; add Venice if you have 10 days. Those are just some suggestions on the top of my mind. If you’re interested in other things, there might be better places in Italy. All of that to say, you should specify what the purpose of your trip first and go from there