"Holding pen" is a term that connotes captivity, restraint, and confinement. Antonyms for this term would be words that suggest the opposite, like freedom, mobility, and release. Some antonyms that come to mind include "open field," "free-range area," "spacious enclosure," or "large habitat." These terms imply spaces that are open, expansive, and offer ample room for movement and exploration. Unlike a holding pen, they suggest a place that does not limit or bind an entity. Thus, using these antonyms can help convey a sense of liberation and freedom that counters the connotations of "holding pen.