Place first three ingredients in a medium bowl. Press firmly on solids with a muddler or the back of wooden spoon until mashed.
Mix in tequila, lime juice, Simple Syrup, and Cointreau, then 1 cup ice.
Stir to blend well.
Divide remaining ice between two tall glasses.
Strain Margarita mixture into glasses.
Garnish with watermelon skewers and mint sprigs.
Note: To make your own Simple Syrup, stir together one cup sugar and one cup water in a saucepan over medium heat. Bring to a boil, then reduce heat and simmer about three minutes, until sugar is dissolved. Cool before using.