Antonyms for the phrase "cools one's heels" could include "acts decisively," "moves quickly," or "takes immediate action." Rather than idly waiting, these antonyms suggest a sense of urgency and proactivity. Other antonyms could include "proceeds immediately," "dashes off," or "jumps into the fray." These phrases all convey a sense of motion and energy, rather than the passive waiting implied by cooling one's heels. Ultimately, the choice of antonym will depend on the context in which it is used and the tone that the writer or speaker is attempting to convey. For a sense of urgency, bold action is often the preferred choice.