Spread 1/3 cup rice evenly onto each nori sheet, leaving a 1-inch empty space at the far end of the sheet. Toward the center of each sheet, line up the following ingredients, without overlapping: 3 heaping tablespoons grape halves, 2 slices avocado, 6 cucumber matchsticks, and 2 heaping tablespoons crab.

Using the nori to hold it together, roll everything up tightly. Cut each roll into 8 pieces. Serve as-is or with the optional toppings.