Potatoes, carrots, onion, and celery are all pretty standard stew additions because they impart savory flavor and hold up well when cooked. Mushrooms are also a popular stew addition, although I personally prefer my stew without them. Other root vegetables like turnips or parsnips would be good choices for adding to your stew, and you could even experiment by adding broccoli, cauliflower, cabbage, leek, or even kale as substitutes if you have some on hand that you want to use up.