General synonyms:
* Act
* Doing
* Activity
* Movement
* Operation
* Performance
* Process
* Procedure
* Work
Synonyms related to specific types of action:
* Behavior: for actions related to conduct or behavior
* Effort: for actions requiring physical or mental exertion
* Reaction: for actions responding to a stimulus
* Response: for actions taken in reply to something
* Step: for actions that are part of a larger process
* Task: for actions that need to be completed
Synonyms with a more formal tone:
* Execution
* Implementation
* Intervention
* Maneuver
* Measure
Synonyms with a more informal tone:
* Goings-on
* Happenings
* Stir
* Hustle
* Move
The best synonym to use will depend on the specific situation and the meaning you want to convey.