Heat oven to 425 degrees F. Heat a large pan over medium heat and add olive oil and grated sweet potato. Cook for about 2 minutes and then add grated zucchini, salt, pepper and spices. Stir together and cook until vegetables are tender, about 5 minutes. Lay tostada shells on a large baking sheet and spread each with about 1/4 cup refried beans, 1/6 of the veggie mixture and top with 2 tablespoons shredded cheese. Bake tostadas until heated through and cheese is melted, about 5 minutes. Add your favorite toppings and enjoy!