It would appear that more often than not I forget some critical items while packing. This list is an attempt to counteract that, while providing some explanation for my selection.
Even when traveling to a country without mobile coverage an Android device is quite useful. A few critical apps include:
For the device itself, features I would look for in choosing a device for travel are: removable battery, microSD card, unlocked, GSM radio, and LTE radio (as many bands as feasible). Unfortunately I have not found this device. Really any device is fine if you don't need to make calls.
Additional batteries for your phone are a nice luxury. Between using third party replacement batteries or an external pack I prefer the replacement batteries; they are lighter and give your phone a full charge in minutes. I do have a solar powered external pack for camping, but I don't find myself using it too often.
Cables to attach your phone to your rental car, namely a charging cable and an aux in cable for music. Charging cable is even more important if your phone is being used for a GPS.
Outside of the obvious selection of clothes (what you need to do your job), I suggest clothes made of nylon, polyester, or rayon. These are fast drying and pack much lighter and smaller than cotton. Even for work trips where they aren't worn during the day they make excellent backup changes of clothes.
Laundry soap, particularly the pre-measured powdered packets (only one or two). Laundry is another benefit of the fast drying materials, as washing one garment in the sink and having it dry by morning is feasible.
Additional accessories should include sunglasses, specifically when driving. Also a watch, stressing dependability and low price so it could be lost or stolen without concern. I avoid watches which sync with the atomic clock, as I will second guess it when traveling between time zones. Fun fact: President Clinton and George W. Bush both wore Timex watches while in office.
I believe that if I cannot fit everything in the TSA liquids friendly ziplock bag something is wrong. Do not forget dental floss, it is handy for tying things together in addition to hygiene. I have had trouble finding chapstick when traveling in Asia, and try to keep a spare. Fluoride mouth wash is not a replacement for brushing teeth, but more effective than nothing. Just verify the container used has a good seal and will not leak when upside down.
A double edged safety razor makes carrying many blades easily, and if lost or confiscated the investment is much lower than fancy cartage razors on the market. Shaving soap in a plastic container never gets flagged in an X-ray scanner, while shaving cream canisters look suspicious on the monitor.
A pill bottle can hold multiple kinds of medicine. Trim down blister packs to keep the name and dosage visible along with keeping the pills more sanitary. For stomach issues, I keep bismuth subsalicylate in pill form. For headaches, I keep naproxen and paracetamol (remeber, paracetamol and alcohol do not mix!). Ibuprofen is useful for muscle aches. For a cold having dextromethorphan helps you keep working, in addition to some lozenges and multivitamins. It is easy to keep thinking of medicines to keep with you, however the idea is to pack light, hence my focus on what I've experienced in past travels (colds, stomach aches, hangovers).
Toilet paper is not always available depending on the destination. Not much is needed; keeping one use with you and purchasing more once depleted is viable in any urban environment.
While traveling meals do not always follow a strict schedule, so I try to keep granola bars nearby. Avoid bars which can melt (chocolate), or crumble a lot. When traveling overseas, jetlag often keeps you up at weird hours in a strange city, and this snack can help a lot.
I don't drink coffee, and not all of the world has other caffeinated beverages available. The concentrated caffeinated water enhancer type products help with mornings and many servings pack very small.
Earplugs and blindfold help sleeping on aeroplanes, along with reducing fatigue from the drone of turbojet engines. A winter headband makes a good blindfold, and is useful if the temperature drops.