In a small saucepan, over medium heat, bring the sugar and 1 1/2 cups cold water to a simmer. Cook, without stirring, until the sugar dissolves and the mixture thickens slightly, about 5 minutes. Remove from heat, add the basil, and let cool to room temperature. Strain the syrup into a resealable container and discard the solids. Pour 2 to 3 teaspoons of the basil syrup into each of 8 ice-filled glasses, then fill with lemonade. Garnish with an additional sprig of basil. (You can cover and refrigerate the remaining syrup for up to 2 weeks.)
Tip: You can add a splash of basil-infused syrup to any summery drink, whether ginger ale, iced tea, mineral water, or sparkling wine (such as Prosecco).