My husband can tell you about the days early in our marriage when we would go visit family in Canada and I would have a suitcase and a few additional bags. I made sure to pack everything I thought I would need. Then I would pack additional clothes and supplies for things I may not need now, but in case anything changed, I was prepared.
Fast forward several years. My husband and I were leaving the continent to go backpacking through India. I purchased the smallest hiking backpack, known as a day pack. I planned to only pack what I could carry easily through India. If you can imagine, my pack list shrunk by over 90%. I did so much research for this trip that I had hoped to pack exactly what was needed. Between that trip to India and many future trips, I feel I have packing down to a science (even if I still wait until the last minute to pack).
Having been to 24 countries now, I now can boldly swap out my “must pack” based on the country I am traveling to. To keep things simple, here is a list of “must have's” I would use for any country I would visit. Some of these "must have's” are readily available at a store in the country you are visiting. However, if you are traveling to remote locations in Thailand, backpacking in the countryside of Spain, hiking through the jungles of Belize, I would recommend packing these instead of rolling the dice of how to find them.
Liquids bag
2 oz Campsuds (1 per week): A biodegradable concentrated cleaner for hair, skin, laundry, and more.
Lemon eucalyptus essential oil: For a DIY bug spray.
Travel Sized Toothpaste. Dr. Bronner's is a great line.
Lip balm
Travel sized Deodorant
Travel sized Sunscreen
Contact Fluid 2 oz, if applicable
Dry goods bag
Band aids
Alcohol wipes
Ginger People chewable ginger tablets: great for upset tummies
Boiron Arnica Montana 30C
Boiron Apis mellifica 30C
Renew Life Ultimate Flora Everyday Probiotic Go-Pack 15 Billion (30 caps)
Travel Sized Toothbrush: Radius TOUR™ Travel Brush
Electrolyte Drink Mix: i.e. Ener-C. 7 mini packs. In case of dehydration or illness
Homeopathic allergy supplement. Great options: Allercetin, BioAllers, etc.
Clothes
Flip flops for shared showers and inside locations
Comfortable walking shoes that are suitable for walking long distances, multiple terrains, and is easy to put on and off. (In locations like India, you want them to slip on and off as you will often be taking them off and putting them on.)
Location appropriate clothing for 2-3 days. These will be hand washed.
Underwear for 4-5 days. These will be hand washed.
Misc. supplies
Organic Sambucus Zinc 24 Lozenges bag
Lifestraw Bottle
Empty Spray bottle
$75-$100 worth of currency
Copy of your passport and visa (if applicable)
The Friendly Swede Neck Stash Pouch
A list of emergency contacts
Flat, small snacks that are travel friendly. A great one is Stretch Island Stretch fruit.
Poncho
Rope to line dry cloths
Combination lock
Mini key chain sized flashlight or head lamp
Backpack poncho/wrap
Pen
Print out of your reservations
Hair pick, comb, or brush
Hat
Sunglasses
Campers toilet paper
Prescribed medicine will need to be in a separate bag and will need to contain a note from your physician.
Travelers insurance: World Nomads, REI and other companies offer this.
Optional
Ear plugs: if you are a light sleeper
Eye mask: if you are a light sleeper and especially for plane rides if you don't sleep easily
Mini notebook
Hot Hands and Hot Toes: when going to cold climates
Feminine supplies: difficult to find in outside of major cities in non-European countries
Contacts, Contact Case, and Glasses for sight (if applicable)