Synonyms:
* Positive: Excited, eager, passionate, fervent, ardent, zealous, spirited, animated, vivacious, bubbly, keen, motivated, inspired, fired up, pumped up, raring to go.
* Neutral: Interested, involved, engaged, active, supportive, committed.
Antonyms:
* Negative: Apathetic, indifferent, dispassionate, unenthusiastic, lukewarm, uninspired, bored, detached, disinterested, passive, reluctant, unwilling.
The best synonym or antonym for "enthusiastic" will depend on the specific context in which you are using the word.