没有互联网的多个手机之间传输数据的好方法（现有）是什么？

Background

I have an idea for an app on vacation that needs to communicate to other phones with the same app. While on vacation those phones might not all have internet as roaming can be very expensive. The data is not a lot: like 500 kB max would suffice (in json).I have an idea for an app on vacat