As mentioned earlier, if you don't have an E.U. Driver's License, each driver listed on the rental form will require the compulsory International Driver's Permit for driving in Italy, and it's also highly recommended in France.
Again as mentioned, you'll need a Highway Tax Vignette for driving in Switzerland, which you can normally obtain at the border. If your route from Germany to Switzerland takes you through Austria, you'll also need a Highway Tax Vignette for there. There are hefty fines if you're caught without the Vignettes, with fines usually payable on the spot!
In Italy you'll also have to be vigilant to avoid the dreaded ZTL (limited traffic) areas which exist in many Italian towns. Again, hefty fines for each pass through one of these zones, which you may not know about until several months after you return home. You'll also have to contend with speed cameras including the devious Traffic Tutor system in Italy which measures not only instantaneous speed but also an average between two points. Be sure to budget for tolls on the motorways.
Happy motoring!