Paul and his wife are traveling to Croatia Saturday and he's worried about data roaming charges. Leo says that international data is rapidly changing thanks to T-Mobile's free Edge data plan that started last year. As such, AT&T has changed from offering an expensive data package to a "day pass."
Google Fi is a solid option, because they would pay as they go at $10 a month. If they have a Google phone, then they'll be in good shape.
They could also get a local SIM, which would be cheaper, but then they'll have a local Croatian number. Since Paul is staying for a month, that may be the best option. They should check out prepaidwithdata.wikia.com.