1) In a small pan, combine the balsamic vinegar and brown sugar. Bring to a boil over medium heat, stirring constantly until sugar is completely dissolved. Reduce heat and simmer until glaze is reduced by half, about 20 minutes. Let cool.
2) In a large bowl, combine watermelon, goat cheese and arugula. Stir to combine. Drizzle balsamic glaze just before serving.