General synonyms:
* Composition
* Authorship
* Scribbling
* Penmanship
* Composition
* Literature (for written works)
Synonyms focusing on the act of writing:
* Composing
* Drafting
* Penning
* Scripting
* Authoring
Synonyms focusing on the written product:
* Manuscript
* Text
* Document
* Work
* Piece
Other synonyms:
* Epistle (formal letter)
* Narrative (story)
* Essay (formal written work)
* Poem (written work with rhythmic structure)
* Article (written work in a newspaper or magazine)
The best synonym for "writing" depends on the specific situation and the intended meaning.