In a small bowl, whisk together the lemon juice and vinegar. Whisk in the olive oil slowly. Add salt and pepper, to taste.
ASSEMBLE THE SALAD:
In a large bowl, toss the arugula with the watermelon. Add the vinaigrette (as much as you'd like, to taste), and toss. Sprinkle in the feta cheese and pine nuts and gently toss again. Serve immediately.
*To toast the pine nuts, heat a skillet to medium. Add the pine nuts to the pan and stir often until they begin to turn golden. Remove the pine nuts to a bowl or a paper towel to cool.
Goat cheese may be substituted for feta. Mixed greens may be substituted for the arugula.