Broccoli stems can be sliced and used in coleslaw, a stir fry, or even in a salad. Celery leaves can also be used in a salad. When making stuffed mushrooms, dice stems and put back into mushroom. When making stuffed peppers, keep crowns for something else.
I love to freeze parts of veggies I'm not using (keep cruciferous veggies like broccoli separate for other uses) and use them to make vegetable stock for soups!