Heat a grill to high heat. Oil the grill grates with a towel soaked in canola or vegetable oil.
Put the grapes on a perforated grill sheet or in a grill basket and grill over high heat, tossing occasionally, until they just start to burst, about 6 minutes. Transfer to a plate to cool completely.
Meanwhile, grill the peach wedges over high heat, turning once, until lightly charred, about 6 minutes. Transfer to a plate to cool completely.
Combine the wine with the simple syrup, tequila and grilled fruit and stir well. Refrigerate until the sangria is chilled and the flavors are blended, at least 1 hour and up to 8 hours. Serve the sangria over ice.
You can also cool the grilled fruit, then freeze it to help keep your sangria extra cold.
Cook's Note: To make simple syrup, combine equal amounts of sugar and water in a small saucepan and bring to a boil; simmer, stirring constantly, until the sugar is completely dissolved, about 3 minutes. Remove the syrup from the heat and let cool completely. The simple syrup can be refrigerated in a glass jar for up to 1 month.