In a small bowl, stir together sugar and orange juice.

In the container of a blender, pulse together half of sugar mixture, half of watermelon, and half of mint leaves until smooth. Strain through a fine-mesh sieve into a large pitcher, discarding solids. Repeat process with remaining sugar mixture, remaining watermelon, and remaining mint leaves. Refrigerate until cold.

Stir in cold club soda just before serving. Garnish with orange, watermelon, and mint, if desired.