The word "grimfaced" refers to a facial expression that is serious, stern, and unhappy. Antonyms for "grimfaced" would therefore be words that describe a facial expression that is the opposite of this. Some possible antonyms could include words like "smiling," "grinning," "laughing," "merry," "jovial," "cheerful," "playful," "happy," "amused," or "lighthearted." These words suggest a more jovial, light-hearted demeanor, and are the exact opposite of the serious, stern, and unhappy expression conveyed by the word "grimfaced". By using antonyms to describe someone's facial expression, you can convey a variety of emotions and create a more nuanced picture of their personality.