Using a mortar and pestle, crush or muddle some fresh mint. Spoon in the watermelon and pineapple cubes, and crush them bit by bit. Toss in the lime quarters, and continue mashing. Then, in a cocktail shaker, add 1/2 cup ice cubes, all of the fruit and mint mojito mixture, then add the rum, pineapple juice, and finally, a dash of sugar cane syrup. Shake well to combine. Divide between 2 hefty glasses garnished with lime wedges and mint sprigs.