I make broth in the slow cooker, at least overnight (up to 24 hours) from the leftover chicken, beef or lamb bones. I always put in celery stalks and leaves, carrots, an onion (cut in half and chargrilled, except for chicken), whole peppercorns, dried or fresh parsley and salt. To the chicken stock I also add a few cloves of garlic.
All of the above gets strained out of the finished broth, though. I then use it to prepare soups, usually by adding sliced mushrooms, fresh garlic and/or ginger, chilli pepper flakes and soya sauce. To make it more filling, I'll add some prawns or shredded cooked chicken. Might stir a tablespoon of Greek yogurt into my bowl as well, or squeeze some lemon into it.
Another way I use the stock is to boil some leftover greens (spinach, kale, rocket, watercress..) in it until soft, which is less than 10 mins, then blend it with some cream, yogurt or cream cheese. Yummy!