General:
* Stream
* Brook
* Run
* Rivulet
* Gulch (especially if it's narrow and steep)
* Watercourse
Informal:
* Branch
* Hollow
* Ditch
* Drainage
More specific:
* Bayou (a creek in a swampy area)
* Draw (a dry creek bed)
* Wash (a shallow, dry streambed)
* Ravine (a deep, narrow gorge)
The best synonym for "creek" will depend on the specific context and the nuance you want to convey.