If you mean "unable to relax or be still", then some opposites include:
* Calm
* Peaceful
* Tranquil
* Serene
* Still
* Relaxed
* Composed
If you mean "constantly moving or changing", then some opposites include:
* Stable
* Steady
* Unchanging
* Constant
* Settled
* Static
If you mean "uneasy or anxious", then some opposites include:
* Content
* Satisfied
* Happy
* At ease
* Relaxed
* Calm
The best opposite word will depend on the specific meaning of "restless" in the context.