Refresh with one of these 90-calorie virgin margaritas. Add to a blender with ice for a frozen adaptation of this drink.
Working in batches, combine lime juice and agave nectar in a blender, then blend in 2 cups seltzer and ice. Transfer to a pitcher and stir in 4 more cups seltzer. Stir in tequila, if desired. Garnish with lime slices and serve cold.