Contrasting Seriousness:
* Serious fun
* Solemn playfulness
* Earnest amusement
Contrasting Order:
* Chaotic joy
* Organized chaos
* Structured spontaneity
Contrasting Intensity:
* Gentle violence
* Subtle exuberance
* Soft intensity
The best oxymoron for "playful" will depend on the specific context and the nuance you want to convey.