In a small saucepan, bring sugar, water, and orange zest to a boil over high heat. Reduce to a simmer and cook until sugar completely dissolves, about 3 minutes. Let syrup cool in a bowl.
In the meantime, place 1 tablespoon lime juice on a small plate and 1/4 cup red or pink sugar crystals on a second. Dip the rim of four glasses in lime juice, then in sugar to coat.
In a blender, puree 2 cups of watermelon cubes until smooth.
In a pitcher, combine watermelon puree, syrup, and lime juice. Mix well. Add soda and stir very gently.
Fill sugar-rimmed glasses with ice, and pour watermelon drink over top.
Garnish with lime wedges, watermelon wedges, or mint sprigs.
You can substitute any fruit for the watermelon in this refreshing and summery drink. We garnished ours with red sugar crystals, but I think it would be nice garnished with a sprig of mint and a wedge of watermelon, too. How about a scoop of ice cream? Heavenly! For the summer, you could pour this drink into popsicle molds and freeze it. So many ideas!