Cover a bamboo rolling mat with plastic wrap. Cut nori sheets in half crosswise. Lay 1 sheet of nori, shiny-side down, on the plastic-covered mat. Wet your fingers with water and spread about 1/2 cup of the rice evenly onto the nori. Sprinkle the rice with sesame seeds. Turn the sheet of nori over so that the rice side is down. Place 1/8 of the cucumber, avocado, and crab sticks in the center of the sheet. Grab the edge of the mat closest to you, keeping the fillings in place with your fingers, and roll it into a tight cylinder, using the mat to shape the cylinder. Pull the mat away and set aside. Cover with a damp cloth. Repeat until all of the rice has been used. Cut each roll into 6 pieces. Serve with pickled ginger, wasabi, and soy sauce.