Pour the sugar and water into the sauce pan and cook over medium heat until the mixture just starts to boil and the sugar dissolves. Turn off the heat. Allow to cool, then pour into a jar to save for other cocktails. Store in the refrigerator.
Cook the chopped rhubarb and ½ cup of the simple syrup in the sauce pan over low heat until softened, about 5 minutes. Cool. Combine the rhubarb mixture with the tequila, Grand Marnier, and ice in the blender and blend until smooth. Pour into glasses and garnish each with a basil or mint leaf.