General synonyms:
* Ardor: A strong feeling of passion or excitement.
* Enthusiasm: Intense and eager enjoyment, interest, or approval.
* Fervor: Intense and passionate feeling.
* Passion: Strong emotion or feeling.
* Vigor: Great strength or energy.
* Eagerness: A strong desire or interest.
* Dedication: The quality of being devoted to a task or purpose.
* Drive: A strong inner force that propels someone to act.
* Spirit: A person's mental or emotional state.
More specific synonyms:
* Relish: Great enjoyment. (Often used with activities)
* Avidity: Extreme eagerness or enthusiasm.
* Industry: Hard work and dedication.
* Assiduity: Great care and perseverance.
Antonyms:
* Apathy: Lack of interest, enthusiasm, or concern.
* Indifference: Lack of interest or concern.
* Laziness: Disinclination to work or exert oneself.
The best synonym for "zeal" will depend on the specific context of your writing.