✓ Call your bank(s)
You MUST let your bank know of your travel plans if you plan on using a credit or ATM card abroad. If you don't they will be very helpful and shut your card down thinking it has been stolen and absconded to a foreign country!
✓ Set your email auto-reply
If you will not be checking your work and/or personal email, set up an auto-reply that tells people you are away.
✓ Change your voicemail greeting
Depending on your cell provider plan, you will likely not be using your phone unless you buy a local SIM card at your destination. Some people like to keep the message vague like "My phone will not be in service from x to y...." so it's not obvious that your home is empty.
✓ Switch to airplane mode
Change your phone to airplane mode while you are still in your home country. That way you don't risk turning your phone on when you arrive and having it automatically download all of your emails and incurring hefty roaming fees! If you are not buying a local SIM, keep your phone in airplane mode until you get home. You can still use WiFi and GPS in airplane mode. Sometimes I just remove the SIM card altogether to be safe but that's not as easy with newer phones. There are other precautions you can take such as setting your phone not to use data when roaming, etc.
✓ Copies of passport in luggage and at home
Hopefully you will never need them, but leaving a copy of your passport both at home and in your luggage (separate from your actual passport) will make it much easier to get a replacement passport if yours ever gets stolen.
✓ Credit card info
Either print out and/or put digital copies on your phone of credit card numbers along with the corresponding financial institution phone numbers in case your cards get lost or stolen. Make sure you are aware of foreign transaction fees, ATM fees, and possible ATM reimbursements for each of your cards.
✓ Emergency cash
I keep about $100 USD in cash hidden separately from my other money in case I get mugged.
✓ Print out your first hotel info
Be ready for your first destination. Know how you plan on getting from the airport to your hotel and have the name, address and phone number of the hotel printed out. If you can print it out in the local language (some reservation sites such as Booking.com have this option built in), even better. That way the bus or taxi driver can easily get you to where you want to go.
✓ US Citizens register with STEP
The Smart Traveler Enrollment Program (STEP) is a free service to allow U.S. citizens and nationals traveling and living abroad to enroll their trip with the nearest U.S. Embassy or Consulate.
✓ Download offline maps
Maps.me and Google Map apps allow you to use these GPS maps while you're on the road without needing WiFi or cell signal. You just need to download the specific areas ahead of time.
✓ Download offline languages
With the Google Translate app, you won't use any data or WiFi to translate with offline languages. And just wait until the first time you use your camera to translate a sign realtime!
✓ Download airline entertainment apps
Airlines are making the move to using apps for access to in-flight entertainment on your mobile device. But you typically have to download their app ahead of time.
✓ Download copies of any reservation confirmations
I never want to risk not having access to email to show someone where I'm going or prove a reservation. Download these receipts (for hotels, tours, flights, travel insurance, etc.) to your phone (or print them) before you leave to lessen any risk. I also like to keep all trip-related emails organized in a specific sub-folder in my email app to easily find them when I'm on WiFi.
✓ Download guidebooks and travel notes
You don't want to forget to take along all that research you've done while preparing for this trip! Download (or print) guidebooks, travel & language notes.
✓ Weigh your luggage
Make sure you are not at risk of overweight luggage since the airlines charge so much for this now! If you have to, wear your heaviest clothing and shoes.
✓ Triple check that you have your passport
It may sound silly, but I actually had this happen with a friend of mine. We were on our way to the airport and she realized that her passport was still at home in the copy machine! As soon as you start planning your trip, you should make sure you can locate your passport and also make sure it is not expiring soon.
✓ Bring cash as needed
If you need to pay for a visa on arrival in the country you're visiting, make sure you either have enough of whatever type of cash they accept (sometimes they take US $, sometimes Euros are okay, etc.). Hopefully if you needed to apply for a visa ahead of time, you have done that already!
Make sure your arrival airport has an ATM so you can get money for your first transportation. That or bring enough cash to exchange at the airport or even change money ahead of time at home.
✓ Have handy plane goodies
Make sure your inflatable pillow, book, compression socks, headphones, pen, and other essentials are handy in your under-seat bag.
✓ Pack your travel snacks :)
Of course you will want your yummy snacks! Nuts, pre-cut fruit, peeled hard-boiled eggs, snack bars, cookies, cheese & crackers, pb&j sandwiches, are all good things. If you will be in an airport that has free drinkable water (water fountains), bring an empty water bottle to fill once you are through security.