Heat the grill to high. Heat an outdoor gas grill to high, or prepare a charcoal grill for high, direct heat grilling.

Cut the watermelon. Trim the ends off the watermelon. Stand the watermelon on one cut end, cut in half, then cut each half in half again. Cut the quarters into 1/2-inch-thick slices.

Season the watermelon. Combine the sugar, lime zest, and red pepper flakes in a small bowl. Sprinkle the watermelon slices on both sides with the sugar mixture, then rub the sugar mixture in to create an even coating.

Grill the watermelon. When the grill is hot, add the watermelon slices in a single layer and grill until warmed through and grill marks appear, 2 to 3 minutes per side.

Serve. Serve the grilled watermelon on its own with lime wedges, topped with feta cheese or Greek yogurt, or as dessert with a scoop of ice cream.