AT&T is the same $10 plan. As soon as you take your phone out of Airplane Mode the account gets charged $10 for 24 hours. That timer starts from the moment you turn on +24 hours not the calendar day.
We survived for two weeks in France with just Wi-Fi and Whats App. Wi-Fi is everywhere, and most the restaurants and bars have free service for customers. Apple facetime works also.

We also used Google maps for driving and walking around in the city. While we had service I would put in the way points and it downloaded the maps required. You do not require Wi-Fi for GPS, but you do need Wi-Fi for the maps.

The $10 pay as you go works out - if you need it you have it. If you do not need you do not pay.