General synonyms:
* Easy
* Simple
* Smooth
* Seamless
* Natural
* Unforced
* Unstrained
* Fluid
* Graceful
* Relaxed
More specific synonyms:
* For physical actions: Automatic, instinctive, spontaneous
* For mental processes: Intuitive, instinctive, automatic
* For artistic creation: Inspired, spontaneous, unstudied
* For interpersonal interactions: Charming, magnetic, engaging
Informal synonyms:
* Piece of cake
* No sweat
* Breeze
* Walk in the park
The best synonym to use will depend on the specific context and the desired tone.