General Antonyms:
* Cheerful
* Joyful
* Happy
* Bright
* Lighthearted
* Lively
* Merry
* Optimistic
* Upbeat
More Specific Antonyms:
* Whimsical (if somber refers to a lack of imagination or playfulness)
* Colorful (if somber refers to a lack of vibrancy)
* Animated (if somber refers to a lack of energy)
* Celebratory (if somber refers to a lack of festivity)
* Hopeful (if somber refers to a lack of optimism)
The best antonym for "somber" will depend on the specific situation. For example, if you're describing the mood of a room, "cheerful" might be the best antonym. But if you're describing someone's personality, "optimistic" might be more appropriate.