Place watermelon in a blender or food processor. Cover and blend until smooth. Strain watermelon puree through a fine mesh sieve; discard pulp.
In a large bowl combine sugar and mint. Using the back of a wooden spoon, lightly crush mint by pressing it against the side of the bowl. Add grape juice, lime peel, lime juice, and watermelon puree, stirring until sugar is dissolved. Stir in club soda. Serve in glasses over ice. If desired, garnish with mint sprigs and watermelon wedges or balls. Serve immediately. Makes 9 servings.