Summer is a great time for me to collect ingredients for stock in the freezer. For chicken stock, I collect carrots, celery, parsley, and onion. For vegetable stock, I also collect bits of beans and mushrooms.
Currently in my second bag, I have old carrot and celery sticks, stems from parsley that was used to create basil/parsley puree for the freezer, and now bits of onion from the zucchini relish recipe that I started today. Also in the bag are the carrot thinnings that I pulled out of the garden yesterday to make room for the other carrots.