If someone is zestful, they're energetic and enthusiastic. Your zestful performance on the soccer field should earn you the title of Most Valuable Player!
The noun zest has two meanings: the sour outer layer of peel on a citrus fruit or a passionate enthusiasm. The adjective zestful is used only in the second, more figurative way, to describe someone with true zeal or verve. A zestful response to an essay question will definitely get your teacher's attention, and a zestful audition may very well win you the role of Macbeth in your school's upcoming production.