For an extra layer of flavor, consider grilling the peach slices lightly before adding them to the syrup. Experiment with different types of bitters to customize the cocktail to your preferences. Orange or peach bitters would complement the other flavors beautifully. Adjust the sweetness of the syrup to your liking by adding more or less sugar. Remember that the bourbon will also contribute to the overall sweetness. If you don't have lime-flavored sparkling water, plain sparkling water with a squeeze of fresh lime juice will work well.