Google Translate is available offline as well as when you are on wifi or a data plan. You can download the languages of your choosing, now without taking up so much space on your phone. This is a great feature when faced with a menu full of unfamiliar choices...or when attempting public transportation in a foreign language.
Museum apps are a great tool when you are visiting a new museum, or revisiting a favorite. This article in the NYTimes reminded me how great the museum produced apps can be. Many of them do not require wifi if you download them before visiting, or the museum may even have wifi available for their app. Check museum websites as you are planning your trip! The Louvre, the British Museum and the Museum of Modern Art (NYC) are among the museums offering an app.
Keep your devices charged! I use an Anker charger that is big enough to charge both an iPhone and my iPad, but the small ones that will give your phone a full charge are well worth the small cost in case you have an emergency. Here are a few examples-
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