Formal:
* Polishing
* Refinement
* Enhancement
* Embellishment
* Perfection
* Acme
* Cui bono (Latin for "to whom is it a benefit?" - implies a final touch that makes something truly complete)
Informal:
* Tweak
* Touch-up
* Last minute detail
* Finishing kick
* Top off
More specific:
* Flourish (for artistic touches)
* Highlight (for emphasizing a feature)
* Accessorize (for adding accessories)
* Capstone (for something that completes a structure)
The best word to use will depend on the specific situation. For example, "polishing" might be a good choice when describing the final stages of writing a manuscript, while "tweak" might be more appropriate for a small adjustment to a design.