Granular laundry detergent
No one expects this when they set out to travel, but the unexpected can always happen! Maybe you spill something on a shirt that will stain forever if not washed out fast, or the weather changed out of the blue and you have to reuse clothing that you already deemed not clean enough. Anything under the sun can happen, but you have to make sure you are prepared for anything, and that includes an impromptu bathroom sink laundry session.
That’s why you should look into taking a small bottle of granular laundry detergent with you. It doesn’t weigh a lot and you can get it in your favorite scent to avoid having to get confused in an aisle of a supermarket in a foreign country, and possibly waste half of that detergent because you have to throw it away. Moreover, since it is in powder form, it is not subject to the 3-1-1 rule at security, so it can just chill in your bag during screening.