Do something zestfully, and you'll do it in a strong, spirited way. Don't recite your part in the school play in a dull monotone; try performing it zestfully and the performance will be much more exciting.
This adverb comes from zest and its French root, zeste, "piece of orange or lemon peel used as a flavoring." Zestfully is used for a figurative version of tang or brightness, when something is full of life and energy. When you play zestfully in your big tennis match, you really throw yourself into it — you're energetic and excited about playing.