If using bamboo skewers, be sure to soak them in water for at least 30 minutes prior to grilling, or use metal skewers.
Preheat an outdoor grill to medium-high heat, about 400 degrees F.
Slice 2 medium zucchini into 1/4-inch thick medallions. Cut 2 medium green or red bell peppers into bite-sized chunks. Clean 8 ounces white or cremini mushrooms and keep them whole if they are smaller, or halve them if they are larger. Then cut 1 medium red onion into bite-sized pieces. The key is to keep everything in similar bite-sized pieces.
Combine all the cut vegetables along with 1 pint cherry or grape tomatoesin a mixing bowl. Add in 3 tablespoons olive oil, 1 teaspoon salt, 1/2 teaspoon garlic powder, 1/2 teaspoon onion powder, and 1/2 teaspoon black pepper and toss until everything is well coated.
Thread the vegetables one at a time onto your prepared skewers. Make sure to give just a little space in between each vegetable.
Cook on the preheated grill over medium-high heat, rotating carefully about 5 minutes in, until vegetables are tender and the tomatoes pop, total cook time. Cook for 10 to 12 minutes total. Add more salt and pepper to taste. Serve hot.