This is a great historic Persian treat, that is, in a way, like medieval gatorade.
The base recipe is sugar and vinegar, but the possibilities for added flavors and ingredients are as limitless as your imagination. I’ve included both a basic, traditional recipe, and one of my favorite new versions- strawberry, ginger, mint. The traditional version is nearly clear, and subtly flavored, so if you’re worried about drinking vinegar, don’t be. Sweet from the sugar, it also has a slight acidic tang from the vinegar, but it also has a refreshing fruity flavor. The modern version is wonderfully flavorful. I enjoyed it both hot and cold, as well as a dash of sweetener in green tea.
It would be perfect for the hotter climates, such as a blood orange version in Dorne, or exotic fruit varietals in the Free Cities. Imagine enjoying a large glass of this, cooled with a chip of ice, in the water gardens of Sunspear, or atop the tiered pyramids of Meereen. And because it is concentrated, it would also travel well, making it ideal for dusty travel over dunes for the Martell Sandsnakes.
Other regional varieties could also exist, in Westeros as it did in our world. The ancient Greeks made Oxymel, which is similar to this drink: 4-5 parts honey to one part vinegar. Even the Anglo-Saxons had a version with honey! With some apple cider vinegar, wildflower honey, cinnamon, and vanilla, I think one could go far. :)