For describing something very intense or severe:
* Moderate: A moderate climate is neither too hot nor too cold.
* Mild: A mild illness is not serious.
* Temperate: Temperatures are temperate in the spring.
* Balanced: A balanced diet includes a variety of foods.
* Normal: It's normal to feel tired after a long day.
For describing something very far away from the center or standard:
* Central: The central part of the city is bustling with activity.
* Average: The average person spends about 8 hours sleeping per night.
* Typical: It's typical for children to be curious.
* Moderate: A moderate political viewpoint is not too left-leaning or right-leaning.
* Standard: The standard size for a car is not too small or too large.
For describing something very unusual or unusual:
* Ordinary: This is an ordinary day, nothing special.
* Common: It's common to feel nervous before a big presentation.
* Typical: It's typical for people to get tired in the afternoon.
* Normal: It's normal to feel happy when you get good news.
The best antonym will depend on the specific meaning of "extreme" in the context.