Gently muddle 1/2 cup blueberries, 1 thyme sprig, and 2 Tbsp. simple syrup in a cocktail shaker. Add 2 Tbsp. fresh lemon juice and 1/2 cup crushed ice; shake vigorously for 10 seconds. Strain into a 10-oz. glass filled with crushed ice. Top with a splash of sparkling water or club soda; stir gently. Garnish with blueberries threaded onto a toothpick and a thyme sprig, if desired.