General opposites:
* Sad
* Depressed
* Gloomy
* Melancholy
* Downcast
* Dour
* Sullen
* Grumpy
* Unhappy
* Miserable
More specific opposites depending on context:
* Solemn (opposite of lighthearted)
* Serious (opposite of lighthearted)
* Angry (opposite of positive and lighthearted)
* Anxious (opposite of carefree)
* Pessimistic (opposite of optimistic)
It's helpful to consider the specific situation to determine the most appropriate opposite.