General Antonyms:
* Lenient
* Mild
* Gentle
* Kind
* Compassionate
* Forgiving
* Tolerant
* Understanding
* Soft
* Flexible
* Easygoing
More Specific Antonyms:
* Relaxed (for a stern expression)
* Playful (for a stern demeanor)
* Humorous (for a stern tone)
* Indulgent (for a stern attitude)
* Accommodating (for a stern approach)
The best antonym for "stern" will depend on the specific context in which it is used.