The phrase "down the mouth" typically refers to a negative or sad expression on one's face. Antonyms for this phrase could include terms that describe a positive or uplifting expression, such as smiling, beaming, or grinning. Other antonyms may include words that describe an overall positive demeanor, such as cheerful, optimistic, or elated. Choosing to use these antonyms in place of the phrase "down the mouth" could help to convey a more positive tone in written or spoken communication, while also avoiding the use of potentially negative language.