Cut watermelon in half crosswise. Cut a thin slice from the bottom of one watermelon half so that it will sit flat, being careful not to cut through melon. (Reserve remaining half of watermelon for other uses.)
Draw 1-inch notches around cut edge of top of watermelon. Cut out notches using a sharp knife, and discard.
Scoop watermelon into balls to yield about 8 cups, leaving a 1/2-inch margin of fruit around rind to form shell; remove seeds.
Combine watermelon balls, remaining fruit balls, and strawberries; toss lightly to distribute evenly. Spoon fruit into watermelon shell. Garnish with mint leaves.