we have driven to Italy two or three times, but never in the MX5, we camp and also like our comfort so have always used a larger car to fit all the junk in! However we are planning to go again next year, still camping, but to take the MX5.
We tend to drive a day, stay two nights, then drive another day etc, mostly because it saves on having to pack all of our kit away every morning, but also because that way we get to explore a bit around our stopovers. We do tend to take three weeks though to make a real trip of it! We also avoid motorway driving in both France and Italy, not just because we are stingy and do not like paying the tolls, but mostly because we find it very boring.
Our favourite stopover on the way through France is at Argentiere (about 10 miles ‘uphill’ from Chamonix), no particular specific attractions, but stunning scenery, easy access to two glaciers (Argentiere and Mer de Glace - one by cable car the other by mountain train).
in Italy we like Lake Como - again for the scenery, but there are some great driving roads around it too, we stay at a cpmpsite at Lenno, there are hotels too, and at considerably cheaper rates than those over the Lake at Bellagio! Not too far from Mona either.
We have also driven past (but not gone into - we were short of time) the Ferrari museum at Maranello, and their test rack and souvenir shop nearby - great if you are a fan of the marque!
Furthest South we have stayed and the reason for our first trip is Pisa - it is good day’s drive South of Milan, but getting to the leaning tower was worth it for me, lots of good food in that area too, and the Leonardo da Vinci museum at (of all places!) Vinci is nearby, as are lots of fantastic Tuscan villages full of old towers and fortifications, and the city of Siena - also worth a visit.
If we do go again next year we will be heading for the Stelvio pass - reputed to be one of the worlds best driving roads - here: https://maps.google.co.uk/maps?hl=en&ie=UTF-8&tab=wl (hope that works)