The antonyms for "chilling off" are heating up, warming up, intensifying, and escalating. When things are heating up, it means that the situation is becoming more intense and lively. Whereas, when things are chilling off, the situation is becoming more relaxed and calm. Warming up is another antonym for chilling off, and it means that the environment or the situation is becoming less cold or less hostile. Intensifying and escalating also refer to the opposite of relaxing, and they both denote that an event or a situation is becoming more intense, serious or severe. Therefore, the opposite of chilling off can be different based on the context and the intended meaning.