Using a vegetable peeler, peel 6 (3-inch) long strips of lemon zest, taking care to not peel into the white pith. Squeeze 2 tablespoons of juice from the lemon into a small pot. Add the zest strips, sugar and water and bring to a boil over medium heat. Cook, stirring, until the sugar is dissolved. Remove from the heat and cool.
Strain into a serving pitcher and top with the grape juice and seltzer. Serve over ice with slices of lemon, if desired.