Simple and Direct:
* The door opened automatically when I approached.
* The lights turned on automatically as I entered the room.
* The system automatically adjusts the temperature based on the weather.
More Descriptive:
* The car's brakes automatically engaged, preventing a collision.
* The website automatically saves your progress as you work.
* The software automatically updates itself to the latest version.
Figurative:
* Her laughter came automatically, a natural response to his joke.
* The news spread automatically, like wildfire through the town.
* His mind automatically went to the worst-case scenario.
The best sentence for "automatically" will depend on the specific context you're using it in.