General Antonyms:
* Cheerful: This is a common and direct antonym.
* Happy: Similar to cheerful, it signifies a positive and joyful state.
* Bright: This suggests a lighthearted and optimistic disposition.
* Pleasant: This conveys a positive and agreeable demeanor.
More Specific Antonyms:
* Jovial: This emphasizes good humor and sociability.
* Animated: This suggests a lively and energetic personality.
* Enthusiastic: This conveys a passionate and positive attitude.
* Contented: This refers to a feeling of satisfaction and peace.
The best antonym for "sullen" will depend on the specific context and nuance you are trying to express.