Heat grill or grill pan to med-high heat. Spray or lightly oil Watermelon and Brussels sprouts and toss gently in bowl. Remove melon and sprouts from bowl and place on hot grill or grill pan, with sprouts face down and lightly season with salt and pepper. Let sit to achieve grill marks and warm through. If you're using a grill pan, you can warm and brown the chickpeas as well.
Arrange large serving bowl with spinach, chickpeas, cherry tomatoes, avocado and blueberries.
Once watermelon and Brussels sprouts are grill marked, add to large serving bowl and top with feta if using. Season to taste. Serve immediately.