Antonyms for the phrase "with straight face" would be "with a grin," "with laughter," "with a smirk," or "with amusement." These antonyms convey a sense of lightheartedness or humor, as opposed to the seriousness or stoicism associated with a straight face. Other antonyms might include "with a frown," "with a scowl," or "with a pout," which suggest displeasure or anger. Choosing the appropriate antonym depends on the context of the situation, as well as the emotional state and intentions of the speaker. Ultimately, the antonym should provide a contrast to the original phrase and express the opposite sentiment.