The phrase "livens up" implies adding energy, excitement, or brightness to something. Antonyms for this expression may include phrases like dampens down, deadens, or dims. These terms all indicate the opposite effect of liven up, suggesting a reduction in energy, excitement, or vibrancy. Other potential antonyms for "livens up" might include quietens, calms, or soothes. These phrases suggest a calming or relaxing effect that is diametrically opposed to the vibrant energy associated with livening up. Ultimately, the antonyms for "livens up" depend on the context of its usage and what is being described.