Rome to Venice by train, or the south by car
A long weekend in Rome, Venice or Florence fits in the 3 GB pack, which runs for 15 days. The classic first trip, Rome, Florence and Venice joined by high-speed trains over eight to ten days, is a 5 GB trip: the data goes on train tickets, timed entries and walking maps, and that pack lasts 30 days.
The south asks for more. A week or two driving the Amalfi Coast, Puglia or Sicily, with the phone navigating, finding somewhere to park and booking an agriturismo for the night, is where 10 GB stops you rationing. Take 20 GB for a month in the country, a language course in Florence or Bologna, or a laptop working off the phone between towns. The 5, 10 and 20 GB packs all last 30 days.