* Upright: This is a general antonym, suggesting a standing or vertical position.
* Erect: Similar to upright, but often implies a more formal or dignified posture.
* Standing: A more direct opposite of lying down.
* Active: This suggests movement and energy, the opposite of being relaxed and still.
* Sitting: While not directly opposite, it signifies a different posture than lying down.
The most suitable antonym will depend on the specific situation in which you are using the word.