General Restlessness:
* Uneasy
* Fidgety
* Nervous
* Agitated
* Anxious
* Wriggly
* Discontented
* Unquiet
* Excited
* Impatient
Restlessness due to Boredom or Dissatisfaction:
* Bored
* Listless
* Wearied
* Dispirited
* Unmotivated
* Unfulfilled
Restlessness due to Physical Discomfort:
* Uncomfortable
* Irritable
* Aching
* Itchy
* Painful
Restlessness due to Mental or Emotional Distress:
* Distressed
* Troubled
* Anxious
* Worried
* Haunted
Restlessness due to a Desire for Movement or Change:
* Adventurous
* Rambling
* Wandering
* Erratic
* Unstable
The best synonym for "restless" will depend on the specific situation and the meaning you want to convey.