Whenever I travel, I bring along the Sea to Summit Ultra-Sil Day Pack. It weighs 2.4 oz (my iPhone 6 Plus weighs 6.2 ounces) and zips up into a bundle smaller than my fist. But it holds 20 liters of stuff, and I used it every day to carry water, snacks, sweaters, an iPhone charger, a portable wifi, groceries, and things Carla and I bought while walking around. The material feels indestructible.
I also found out that Sea to Summit makes an even smaller lighter daypack called the Nano, an 18-liter capacity day pack that weighs 1.1-ounces and "compresses to the size of an egg."