Preheat grill to 550°F.
Place watermelon slices on hot grates and grill 3 minutes per side.
Remove watermelon from grill and slice into cubes 1 1/2-inches-thick.
In a saucepan, combine balsamic vinegar and honey. Bring to a boil and cook for 5 minutes.
In a large salad bowl, combine arugula, feta cheese, pine nuts, thinly- sliced red onions and grilled watermelon cubes. Gently mix and add balsamic glaze to taste.