General:
* Adornment
* Embellishment
* Ornamentation
* Trim
* Garnish
* Enhancement
* Dressing
* Beautifying
More specific:
* For a room: Furnishing, interior design, styling
* For a person: Attire, finery, apparel, costume
* For an object: Inlay, engraving, etching
The best alternative depends on the context and the specific type of decoration you're referring to.