Toss first 3 ingredients with 1/4 cup Lemon-Poppy Seed Dressing.
Layer watermelon, peach mixture, honeydew, berries, and grapes in a large glass trifle dish or tall glass bowl. Serve immediately, or cover and chill up to 8 hours. Toss with remaining 1/2 cup dressing just before serving. Garnish, if desired.