The words that can be used as antonyms for the phrase "cause ennui" are those which are related to excitement, stimulation, and enthusiasm. Some possible antonyms include "stimulate," "enthrall," "amuse," "fascinate," "captivate," "inspire," and "interest." These words can be used to describe actions, events, or experiences that evoke positive emotions and keep a person engaged and entertained. Unlike the feeling of boredom that is associated with ennui, these words refer to a state of being energized and enthusiastic, where one is fully engaged and motivated to continue participating in a given activity or experience.