I have an iPhone, so I’m biased, but most of this will apply to other smart phones as well.
WiFi
American college students who have studied abroad know that one of the first things you do when you go into a new establishment (cafe, restaurant, bar) is ask for the wifi. It’s a practice that I continue to this day. If you opt to not purchase an international call/text/data plan while traveling, this is how you’re going to be able to communicate with friends, family, and the outside world. The great thing about iCloud keychain, is once you enter in the password to the wifi on your phone, it’s there forever (assuming the router is still up and running and the password hasn’t been changed). Whether you’re going back to the same place the next day, or you’re visiting years later, you will have a point of reference where you now know you can get access to the internet. It may be annoying, but you build up an international list of places that you know you will be able to access the internet from, and I can tell you from personal experience I have returned to places years later and I still connect to the wifi automatically. Many public places also tend to have free wifi that will require one step authentication (giving them your email address).
Battery
Nothing is worse than having your phone die while you’re out traveling. It’s harder to get directions, you lose the ability to take pictures, and you lose your form of communication with your travel companions. I always suggest turning your phone on low battery mode after your phone hits 50%, but one way to slow down battery drain even more is by keeping your phone on low battery mode even when its above 90%. This will further increase the amount of time your phone can go without needing a re-charge. I also always encourage people to take an “iPad” charger with them. It’s noticeably different than a regular iPhone charger in that it has the fatter base. Or, use the new 20W USB-C Apple charger which charges your device at lightening speeds. While it’s not good to use this charger to charge your iPhone on a regular basis (it can destroy the battery), it does recharge your iPhone quicker and so if you need to quickly recharge your iPhone while traveling it becomes handy.
Transit Mode
Using your credit card or cash to buy metro cards in other cities around the world can be annoying. Being able to tap and go with Apple Pay is convenient, and enabling Transit Mode makes it even more convenient. By enabling Transit Mode, you no longer have to use facial recognition or biometric unlocking to accept the apple pay transaction which allows you to quicker accept the charge and move through the turnstile quickly.
Location Sharing
iPhones have the ability to share locations from each others phones. I highly recommend that you choose to share your location with all of your traveling partners before you go on vacation. If you’re traveling solo, share this location with a loved one or trusted friend. It has many obvious convenience and safety benefits, and you can always choose to remove the location sharing when you return home from your trip. Again, I can tell you from experience it is very handy.
Shared Notes Folder
Everyone has likely used the notes app before, but have you ever tried creating a shared note? You can choose to “share” a note with multiple iPhone users. This allows everyone to edit the note similar to a “Google Doc” which is perfect for creating a pre-departure checklist, or jointly working on the itinerary for the trip.
Shared iCloud Album
One of the most annoying things about coming back from a trip is exchanging photos. Not anymore. Similar to the notes app, you can create a “shared” iCloud album in which you can invite multiple users to join the group. Every individual can upload their own photos to the album, and the group will have access to the entire album instantaneously. No more hunting down friends for photos.
Currency Converter
On a final note I recommend downloading a currency converter app, such as XE, so you can track how much your spending, or quickly convert how much cash you should take out from an ATM.