Organic Maps for offline navigation

Doesn't base android have double clock when abroad?

Shattered pixel dungeon for endless offline gaming during the flight

Suntimes (foss) for sunrise/sunset. Shows when is golden hour, which is useful for photography.

Windy for weather, i bought premium which costs 1€/month but provides hourly updates on weather.

OsmAnd+ (free) for offline maps. Kind of steep learning curve, but is the most feature rich app I used. You can turn on to show publicly available hiking/biking routes from national parks, plan a route, etc. I used it for hikes and was able to locate water filling stations, cafes, even when internet was not available. Really helpful to plan your walks without needing an account.

Revolut for banking, best currency exchange rates and affordable eSIM in same app. I use it on first day on arrival before I get local sim if i need one, you get 100 mb for free, so its enough to text your booking on first day. Bring separate bank card in case ATM swallows your main card so youre not left stranded with no means to pay.

I got a lot of use out of Google Translate for text.

The Line messaging app is ridiculously popular in Japan, to the point where people sometimes are genuinely confused by someone trying to explain that they don't have Line on their phone:

https://en.wikipedia.org/wiki/Line_%28software%29

For currency converter you can use Converter NOW, most of the clocks can show 2 différents times but for the widget don't know sorry (using mlauncher NO widgets.), for the weather use Breezy.

You should use an offline navigation map that's always useful where you're not in a known country and for me that's all.

EDIT : a good messenger like SimpleX