Serves 8
Bring the sugar and 1 cup water to a boil in a small saucepan. Stir until the sugar has completely dissolved. Remove the pan from the heat and allow the sugar water to cool completely.
Fill a large punch bowl or pitcher with 2 quarts of ice, and pour the seltzer water, cranberry juice, rum, lime juice, and sugar water over it. Give the punch a good stir, and finish it off by adding the cranberries and mint sprigs. Garnish each serving with additional cranberries and mint leaves.