An adapter, charger, and USB cable: Wherever you go, you will need to charge, so why not prepare yourself once and for all. A universal charger with a USB port takes care of two needs in one—while an extra USB cable you never remove from your bag is a no brainer (extra long cables are nice for hotel rooms with weirdly-placed outlets). If you want to get really prepared, a mobile battery pack is always handy to have when you’re traveling—especially when flying in an aircraft that still doesn’t have USB ports. Mophie makes great ones; just remember to charge it before you head to the airport.
A small first aid kit: While you hopefully won’t need to use a first aid kit while away, it’s certainly a god-send when you’re in a foreign country and don’t have time to figure out the local word for “painkiller.” A basic bag of essentials might include those much-needed painkillers—along with band-aids, anti-septic wipes, an anti-histamine for allergies, an antacid for street food emergencies, and some melatonin and ear plugs for when jet lag beckons. Just make sure you don’t absent-mindedly add a liquid or cream that you forget about when going through security.
A travel-sized toiletries bag: One of the biggest hurdles for packing carry-on only is the airline liquids restriction. Since it takes a lot of work to either buy or gather up your favorite products and decant them into carry-on size bottles, it makes sense to do this work just once. Then, when you’ve gotten your products lined up, invest in a transparent plastic pouch so you never have to do the disposable ziplock bag switch pre-security again. When you get home, simply refill or replace any products you used while you were gone, and leave the whole kit in your bag ready for next time.
Weather gear: Weather while traveling is unpredictable, and even if you’re flying somewhere warm, the airport or plane you pass through on your way might be arctic. An ultra-light, waterproof jacket and/or lightweight shell layer can do the trick. Many types pack down to nothing and thus won’t be a pain to carry around as a just-in-case option.