It depends on where you go and what are you going to do there.
Passport - check you have the required validity, not only for full stay, but include the extra time required after your return, some don't count validity over 10 years after issuing.
Flight - check whether you can or have to do online check-in in advance.
Further instructions depends on hand luggage X checked luggage. Pay attention to liquids, electronic devices and batteries/power banks.
You might want to find official pages of your airports to have live access to information boards.
Money - don't overthink it. In some localities you can pay by your card practically everywhere, and if it is shorter trip it isn't worth spending much time. When I expect to use mostly the card and want just a little cash, I withdraw it from my card using an ATM in the location.
Travel insurance
Transport - this is relatively important, especially if you plan to use public transport to get from the airport to your final destination. Individual cities can have peculiar apps to pay for your tickets.
In many cities Google maps have valid bus timetables, but not everywhere.
Check for public holidays and general closing times in your destination (if you arrive on Saturday, and shops are closed on Sunday, you want to know about it). Google maps will probably help here.
Check your mobile phone package for international roaming options and prices.
Have a look at Google translate app, learn to use the live photo translation.
Food - find translation of your food key words, especially allergies and things you really hate, so you can check the labels.
Read tap water recommendations for your destination (some localities have great and tasty tap water, some ok but not tasty, some you can't use even for brushing your teeth).