Cut half of watermelon into 1-inch-thick slices. Cut slices into 25 rounds, using a 2-inch round cutter. Scoop just enough melon from tops to form indentations, using a melon baller. Stir together goat cheese, 2 Tbsp. chopped fresh basil, salt, and 1/4 tsp. freshly ground pepper. Spoon into indentations. Chill 1 to 2 hours. Garnish, if desired.