Use them in healthy puddings and cakes. Vegetables like carrots, potatoes, turnips or swedes, cauliflower and parsnips are available most of the year. They're cheap and there's not much waste. Try cooking without peeling. Even some potatoes can be cooked without peeling.
Onions are cheap and add a bit of zing to lots of cooking.
Large open cup, horse or flat mushrooms have a good taste. Although they may cost more than the button ones, you only need a small amount. Frozen vegetables are just as good as fresh and they may be cheaper. You can use them in cooking e.g., in a risotto or pasta dish or as they are. Things like frozen peas or sweetcorn are cheaper, but bags of frozen sliced green and red peppers or sliced mushrooms can be good value as you only need a little in a dish.