Add water, sugar, and thyme sprigs to a small saucepan. Bring to a light simmer. Once bubbles start to appear, reduce the heat to medium-low and simmer for 5 minutes. Remove syrup from heat and steep for 1-2 hours. Remove thyme sprigs and set aside

Add several ice cubes, thyme simple syrup, grapefruit juice, and plain sparkling water to a drinking glass. For more sweet drink add more simple syrup. Mix with spoon. Enjoy.