Positive:
* Cheerful
* Happy
* Joyful
* Bright
* Smiling
* Merry
* Contented
* Pleasant
* Amused
* Jovial
* Vivacious
* Enthusiastic
Neutral:
* Calm
* Peaceful
* Serene
* Quiet
* Neutral
* Indifferent
Negative:
* Angry
* Irritated
* Furious
* Outraged
The best antonym for "sullen" will depend on the specific context.