My wife and I are taking a much needed and long-overdue trip in October, spending ~2 weeks in Greece.

I've been overseas a great deal, but never with smartphones. This time we'd like to use our phones when we're over there, but I'm not sure how to go about doing this.

My wife has an iPhone 5 on Verizon, and I have a Nexus 4 on TMobile. What do we need to do in preparation? I'm especially puzzled about the iPhone.