You can do more than on a port-day certainly, but two days is really the bare minimum for Rome.   Given your contemplated schedule, keep in mind that the Vatican museum (which includes the Sistine chapel) is closed on Sundays (except for one Sunday a month when it is open, free and miserably packed). So if you want to see the Vatican, you'd have to do it on the half day Saturday after you arrive -- which isn't ideal as you'll likely be tired from your flight -- or on the Monday morning