This is probably one of the best pieces of advice we will give you. The most important app you could have to travel with is maps.me. It is a maps app that works offline and takes up very little space as you download only the countries and regions you need.
You can also search for hostels, restaurants, attractions on the map and then it will work out the best route to take.
Since we started travelling, they have also teamed up with booking.com so you can locate hotels and hostels on the app and then it will redirect your to the booking page.
For the majority of our trips we have left our phones on airplane mode to save battery and avoid any unnecessary charges. The best part about this app is that it can still locate you even with airplane mode on!