Focus on lightness and movement:
* Smoke floated on air like a feather in a gentle breeze.
* Smoke drifted on air like a wisp of cotton candy.
* Smoke rose on air like a dancer on the wind.
Focus on impermanence and ethereal qualities:
* Smoke floated on air like a ghost story whispered in the wind.
* Smoke drifted on air like a fleeting dream.
* Smoke rose on air like a memory fading into the sky.
Focus on color and form:
* Smoke floated on air like a silver ribbon unfurling.
* Smoke drifted on air like a swirling watercolor painting.
* Smoke rose on air like a charcoal sketch against the blue canvas.
The best simile will depend on the specific context and the effect you want to create.