Whip up the lemon-mint syrup a week in advance, and this inventive cocktail will be a cinch to put together at your party. It's bright and elegant, with layers of herbs and citrus flavors.
Lemon-Mint Syrup: In a small saucepan, mix sugar, mint, lemon zest and slices, and 1 1/2 cups water. Bring to a boil, stirring until sugar has dissolved. Reduce heat; simmer 30 minutes. Let cool 30 minutes. Strain syrup, and refrigerate until ready to use (up to 10 days). (Makes enough for 12 drinks.)
Cocktail: Pour gin, elderflower liqueur, 2 ounces syrup, and a splash of club soda into an ice-filled glass; stir to combine. Squeeze a lemon slice over the drink, then drop the slice and a sprig of mint into the glass to garnish.