General Opposites:
* Ceasing
* Stopping
* Ending
* Intermittent
* Occasional
* Sporadic
* Temporary
* Finite
* Limited
More Specific Opposites (depending on the context):
* Interrupted: If something is unceasing, it's not interrupted.
* Paused: If something is unceasing, it's not paused.
* Discontinued: If something is unceasing, it's not discontinued.
Example:
* Unceasing: The river flowed unceasingly towards the sea.
* Opposite: The river flowed intermittently towards the sea (due to a drought).
Choosing the best opposite depends on the specific context and the nuance you wish to convey.