To make the simple syrup, combine all of the ingredients in a small saucepan. Bring the mixture to a boil, then reduce heat to low and simmer for 15 minutes or until sugar is fully dissolved. Cool completely.
To make the drinks, place 3 tbsp of the simple syrup plus the mint leaves in the bottom of a glass. Using a muddler or the end of a wooden spoon, muddle the mint and blueberries and watermelon together in the bottom of the glass.
Add the juice of lime, enough ice to fill the glass, plus the club soda to top it off. Garnish each glass with a lime wedge or slice and a sprig of mint. Stir and serve immediately.
