In a small saucepan, add 1/2 cup (125 mL) water to sugar, 4 sprigs basil and lemon zest from half the lemon and simmer over low heat until sugar dissolves. (Omit basil and zest to make a plain simple syrup.) Strain and let cool.
Chill 8 stemless wine glasses or rock glasses. Once chilled, divide ice cubes, basil leaves from 2 sprigs and lemon slices, cooled syrup and lemonade among glasses. Top up each glass with carbonated water.
ADD SPIRIT: Top with sparkling wine or champagne.