Bring 1/4 cup each sugar and water to a simmer in a saucepan. Remove from the heat, add 3 mint sprigs and steep 10 minutes. Discard the mint; add the zest and juice of 1 lemon and a pinch of salt. Toss with 2 cups blueberries and 3 cups cubed watermelon. Let stand 15 minutes. Top with chopped mint.