In a medium saucepan, combine sugar and 2/3 cup water. Set pan over medium heat and cook 2 minutes, until sugar is dissolved, stirring frequently with a wooden spoon. Add orange juice, Grand Marnier (if using), orange zest and bring to a simmer. Dissolve cornstarch in remaining 2 tablespoons of water and add to saucepan. Simmer 3 minutes, until liquid thickens. Remove from heat and add strawberries and grapes. Toss to coat fruit with liquid. Ladle fruit and syrup into dessert bowls and garnish with fresh mint.