General:
* Round
* White
* Cold
* Frozen
* Compact
* Soft
* Smooth
* Wet (if freshly made)
* Hard (if packed tightly)
* Frigid
* Chilled
* Icy
Descriptive:
* Glistening
* Sparkling
* Fluffy (if loosely packed)
* Clumpy
* Lumpy
* Perfect (if perfectly round and smooth)
* Misshapen (if not perfectly round)
* Fragile
* Ephemeral (lasting only a short time)
Figurative:
* Whirlwind (if thrown forcefully)
* Missile (if used for throwing)
* Projectile
* Weapon
* Blast (if thrown with force)
More specific:
* Giant (for a very large snowball)
* Tiny (for a small snowball)
* Snowball (the word itself can be descriptive!)
The best words to describe a snowball will depend on what you want to convey.