Place refried beans in a small saucepan over medium low heat. Allow to heat while you prepare veggies.
Remove the husk from the ear of corn and using a sharp knife, remove the kernels from the cob. Place the corn kernels in a small mixing bowl. Add diced red bell pepper, zucchini, and thinly sliced onion. Drizzle veggies with 1 tablespoon olive oil and mix until all the veggies are lightly coated with oil. Sprinkle 1/2 teaspoon garlic powder and salt and pepper, to taste, over the veggies and mix well.
Pour just enough olive oil into a large frying pan to lightly coat the bottom. Place over medium high heat and when oil is hot, add the veggies. Cook and stir for about 3 or 4 minutes until veggies are just fork tender. Taste and season with additional salt and pepper, if needed. Remove from heat and set aside.
Set your oven to the broil setting. Line a baking sheet with foil and coat it with non-stick cooking spray. Place 4 corn tortillas on the prepared foil and spray them generously with the non-stick cooking spray. Flip them over and spray the other side. Place baking sheet under broiler and watch closely. Flip the tortillas when they have browned on one side. Remove from oven when both sides are nicely browned and tortillas are crisp.
Assemble the tostadas by placing 1/4 of refried beans on one tortilla. Add a heaping spoonful of the cooked veggies and garnish with salsa, avocado, shredded cheese, light sour cream, and cilantro. Repeat with remaining tortillas.