Rinse all fruit under cold water and dry thoroughly in a salad spinner or on clean kitchen towels; chill blueberries and pretzels briefly while you prep other items.

Hull and halve strawberries and pit and halve cherries; place each fruit in separate bowls.

Place a 16×12-inch rimmed tray on a flat surface and, optionally, set a small 3×3-inch silicone mold in the upper-left to define the blue canton.

Pack blueberries into the canton area in a single even layer, pressing gently so they sit snugly and form a dense block of blue.

Create horizontal red stripes across the tray by placing halved strawberries (cut side down) and halved cherries in neat rows, starting next to the canton and working outward; use an offset spatula to nudge fruit into tidy lines.

Place yogurt-covered pretzel pieces and mini marshmallows in the white stripe areas (or keep pretzels separate for travel and add right before serving); tuck mint sprigs around the edges as garnish.

If you packed pretzels separately, add them to the tray within 10 minutes of serving to maintain crunch.