So in ten days we're going to St. Lucia BWI for my daughter's wedding. A lot of the friends who are attending have never been out of the country. So I've been giving this kind of advise.
Yes, you must have a US Passport, they won't let you on the plane without it. Told them that months ago.
Scan the first page of your passport and laminate it. Carry the copy with you around town, not the original. In the UK a US passport is worth $5K on the Black Market and $10K in Eastern Europe.
Buy a set of international electrical plug adapter and get a US multi-tap for your phone, iPad, etc. You can get a set from Amazon for like $13.
Amazon.com: Travel Smart by Conair M-500E Polarized Adapter Plug Set: Electronics
Get the shots the CDC and your doctor recommends.
Best exchange rate is NOT the airport, but the local ATM.
In general, but not in the case of St. Lucia, some local currency is good in case you encounter a toll road that you didn't expect.
Local GPS if possible.
International calling plan on the cell phone so you don't get wacked with $5/minute calls.
Data off on the phone and use Wi-Fi when you can. many people don't know their phone has that feature and get back from a trip with a $2,000 data roaming surcharge.
Make sure your next of kin knows your itinerary.
I always check the CIA fact book online for local geo-political issues when I travel to Eastern Europe and Asia.
https://www.cia.gov/library/publications/the-world-factbook/
A few local customs and taboos like showing your guest the soles of your feet in mid eastern countries is a no-no. (Putting your feet up on the coffee table for example).
I like to know some local language like "where is the bathroom" And " A beer please"
I'm lucky, I have through my employer, a guaranteed medivac card that will pay unlimited $ to get me back to the USA in a medical emergency.
Other suggestions?