Anything in the fridge that needs to be used.
Throw the old veggies in with "the usual" (below) and you have your "end of week" soup.
Soup mix - split peas, barely, beans, lentils; stock (I use Massell "chicken" usually); bay leaves; salt and pepper; herbs - rosemary, bay leaves, parsley, oregano or marjoram, whatever herbs are to hand really, a splash of red wine if it's around. Veggies I like in soup include celery, turnips, parsnips, carrots and potatoes. Onions.
Ditto to the using up old veggies, saving money and freezing. Soup is good food.