Combine the watermelon, scallions, radishes, feta, basil, mint, lime juice, ¼ cup of olive oil, and honey in a mixing bowl. Gently toss to combine. Season with salt and black pepper to taste.
Arrange the arugula in a salad bowl. Lightly drizzle with olive oil. Mound the watermelon mixture over top.
In a small bowl, stir the pistachios, lime zest, red pepper flakes, and remaining 2 tablespoons of olive oil together. Spoon this over the salad. Garnish with any extra herbs and more black pepper.