When you mention "sushi" vegetarian and vegan options may not be the first ones to come to mind, but they are common and offer a refreshing dining experience.
Shiitake
Shiitake is a type of mushroom which has a strong, piquant taste, and is often dried to be used in soup stock and broths. Sometimes fresh or reconstituted shiitake are lightly seasoned with salt or soy sauce, or gently flame-torched to extract their natural juices. They make delicious sushi toppings.