Do you travel with a spare battery for your phone?
On the most recent trip to Europe, I used my phone to store travel information like itineraries and hotel vouchers, navigate cities, collect my thoughts (write!), communicate with friends, access exhibitions, and board trains. Battery power was not what I wanted to run out of.
From my usage patterns, I needed my phone to last at least 10 hours a day, and up to 16 if I am in extended transit.
Combining my existing two-year-old battery with a brand new one brings the combined run time to a about 16 hours which worked really well; even on days with extended travelling on a plane or train.
Unlike a power bank (external battery pack) or charger, a second battery added negligible weight and bulk. And there is no power cord needed. Compared to the other two options, a spare battery also required minimal material to manufacture, ship, package and dispose. Swapping batteries took as long as the phone needed to boot.
I really think using a second battery is an ideal power source when travelling. It really is a shame that so many phone designs deliberately sacrifice this benefit for the sake of a few millimetres of thickness.