Having some basic ingredients on hand makes last-minute cooking much easier. Many recipes call for items that are difficult to find, something that would not normally be in your refrigerator, foods that you do not like, or unfamiliar ingredients. These are some of the staples that I usually have.
Vegetables: onions, carrots, yellow/orange/red peppers, celery, sweet potato (or white or yellow-fleshed potatoes), kale or Swiss chard, spinach, tomatoes, broccoli, or cauliflower. We love Brussels sprouts so I usually serve them at least once a week. Substitute Brussels sprouts with broccoli or cauliflower.