To increase the nutrients, simmer the stock with a strip of kombu.
Other vegetables or scraps you can add that you might otherwise compost or discard: tops of leeks, parsley stems, woody ends of asparagus, shiitake mushroom stems, carrot peels, ends from squashes, etc. You can freeze these scraps until you are ready to make stock. I usually avoid sulfur-containing vegetables such as cabbage, broccoli and cauliflower since their flavors can be overpowering.