Mapping apps like Google Maps can work offline without any mobile signal, but you must download the maps "offline" ahead of time. Start the Google Maps app (or whichever navigation app you are using) and find the "offline" option and download the map of say Tuscany (you can choose the area) - the larger the map you download in Google Maps "offline" the more space it will take on your device. Offline maps in Google Maps expire in 30 days; you'll be asked to download/update them after that, but otherwise, you can download the "offline" area in Google Maps whenever you wish; within 30 days of your trip in Italy would be ideal.

T-Mobile's unlimited 2G roaming data can be a little slow, but it be sufficient for navigation apps. T-Mobile has just recently introduced some new "data pass" options if you wish to buy high speed data access for short periods of time, in case the 2G data turns out to be too slow.