General:
* Labor
* Toil
* Effort
* Task
* Job
* Occupation
* Employment
* Duty
* Chore
Specific:
* Assignment (for a specific piece of work)
* Project (for a large, complex piece of work)
* Operation (for a specific process or activity)
* Function (for a specific purpose or activity)
* Activity (for any kind of work, physical or mental)
Formal:
* Occupation
* Profession
* Employment
Informal:
* Gig
* Grind
* Hustle
Figurative:
* Stint (a period of work)
* Labor of love (work done for pleasure, not pay)
The best synonym for "work" will depend on the specific context. For example, "labor" might be used to describe hard physical work, while "assignment" might be used to describe a specific task given to a student.