Rinse whole carrots. Do not peel. Remove as little of the carrot end as possible. Slice the carrots into 1/2-inch "coins" on a bias. If the carrot is larger than 1 inch around, cut in half lengthwise, then slice.
In a pot over medium-high heat, heat the rice wine vinegar until simmering. Add the sugar and salt. Stir until the sugar and salt have dissolved. Add the carrots to the pot and allow to cook for 3-5 minutes. Remove from heat and set aside to cool. Store the cooled carrots and brine in a covered container and refrigerate for up to 72 hours.
Optional: Flavor the brine with additional ingredients as desired. Some examples include garlic, cloves, dill, celery seeds, star anise, and more.