Check for any updates to entry requirements for all countries on your itinerary right up until departure. There may be digital forms, or uploading of documents to a specific app prior to travel. Check your airline websites for guidance or the State Department website or embassy websites of the countries you are visiting.
Keep copies of your passport you can easily retrieve it if it is lost. Replacement will be much easier with a copy. Check expiration dates to be sure you have 6 months validity beyond your return date to US.
If you have a travel protection plan, take a copy of your summary of benefits and contact info while traveling. Keep all receipts (food, lodging, transportation, medical, etc) and get copies of records at time of service. Getting receipts and records later is nearly impossible. Contact the Travel Protection Plan company immediately for guidance on what services you will receive and for any help/resources they provide if you have a problem while traveling.
ATM's are best for local currency. All-inclusive resorts require some small bills for tips to servers along the way (be aware some resorts forbid tipping and you can get someone in trouble...if in doubt, ask). Most Caribbean and Mexican resorts will accept US dollars, but you may get change in local currency, so take small bills. FYI, Visa and Mastercard are more widely accepted than other cards.
Download Google Translate to your phone (can use without wi-fi or data). Be sure you check on mobile phone fees while traveling to your destination and how to turn off data!
Travel Documents will have contact info if you need help during travel. Remember to contact your Travel Protection Plan also if needed.
Reconfirm any pre-purchased tour the day before so you know exactly where and when to meet, and what to bring (or leave behind)!
Some countrieds forbid the wearing of camophlage clothing, so leave your camo at home or face a trip to the local jail!
There may be city tax or environmental tax payable directly to the resort at check out. It's usually just a few dollars, but it is becoming more common.