In a small saucepan, combine balsamic vinegar and sugar. Heat to boiling and then reduce heat to simmer. Simmer until reduced to syrup consistency; about 15 minutes. Let cool.
On a serving tray, layer watermelon slices, arugula and sliced brie. Drizzle with cooled balsamic vinegar syrup before serving.
Notes
Balsamic glaze can be made in advance and refrigerated until needed.