Make the balsamic glaze: In a small saucepan, combine the balsamic vinegar and honey. Bring to a simmer over medium heat and cook until thickened enough to coat the back of a spoon, about 5-7 minutes. Let cool.
Thread the watermelon cubes and feta cheese cubes alternately onto small skewers. If using toothpicks, make sure they are sturdy.
Arrange the skewers on a serving platter. Drizzle with the cooled balsamic glaze.
Sprinkle with fresh mint leaves just before serving.