General opposites:
* Down: This is the most common antonym and applies to many contexts, like movement, direction, and state.
* Off: This can be used when "up" means "on" or "in operation."
* Closed: When "up" means "open."
Specific examples:
* Up (as in "the price went up") - Down (as in "the price went down")
* Up (as in "he woke up") - Down (as in "he went back down to sleep")
* Up (as in "the volume is up") - Down (as in "turn the volume down")
* Up (as in "the sun is up") - Down (as in "the sun is down")
* Up (as in "he was up for the challenge") - Down (as in "he was down on his luck")
Other words that could be considered antonyms, depending on context:
* Downward
* Decreasing
* Lower
* Less
* Depressed
* Finished
It's important to consider the specific context and the meaning of "up" in that particular sentence to determine the most accurate antonym.