General:
* Cape
* Mantle
* Wrap
* Shawl
* Covering
* Garment
* Robe
* Overcoat
* Jacket (depending on the style)
Figurative:
* Disguise
* Veil
* Mask
* Screen
* Cover
* Pretext
* Excuse
* Facade
Specific types of cloaks:
* Pelerine: a short, circular cloak with a high collar.
* Burnoose: a hooded cloak worn in North Africa.
* Smock: a loose, sleeveless garment, often worn over other clothing.
* Poncho: a cloak with a hole for the head, worn in Latin America.
The best synonym will depend on the specific context in which you are using the word "cloak."