Bring the sugar and ⅔ cup water to a boil in a small saucepan, stirring, until the sugar is dissolved. Remove from heat and add the lemon peel. Let cool; discard the peel.
Combine the lemon syrup, lemon juice, sliced lemon, and 2 cups cold water in a large pitcher. Add 2 cups ice; stir until the ice is melted. (This will quickly chill and dilute the lemonade.)
Serve over ice and top with gin, St. Germain (or another elderflower liqueur), or vodka, if desired.