General:
* The smell of mist is like the faintest whisper of rain on a cool breeze.
* The air, thick with mist, carried a scent like damp earth after a gentle shower.
* The smell of mist was like a soft, clean blanket draped over the world.
More evocative:
* The mist smelled like a whispered promise of magic and mystery.
* The scent of the mist was like the bittersweet aroma of forgotten memories.
* The air hung heavy with mist, smelling like the sweet, earthy perfume of a hidden forest.
Specific:
* If the mist is near a lake or ocean, the smell could be like the salty kiss of the sea.
* If the mist is in a wooded area, the smell might be like the damp, musky aroma of pine needles.
The best simile will depend on the specific context and the overall mood you want to create.