Fill glass with crushed ice. Combine whiskey, lemon juice, and simple syrup in a cocktail shaker. Shake and strain into glass. Top with a splash of seltzer water and garnish with peach slices (and flowers if you're feeling fancy)!
For even more peach flavor, muddle a small slice of peach in the bottom of your glass with the simple syrup before adding ice and the remaining ingredients. A sprig of mint is also fabulous with this cocktail!