Many of the leafy cold-tolerant vegetables can be successfully grown in unheated greenhouse structures during the winter. In addition, root crops such as parsnips, rutabaga, and carrots can be grown, especially if you get them started before the days become short. The growth slows and ultimately stops for a period in winter but plants stay alive, so you simply harvest them as needed.

Spinach, baby lettuce, and arugula could be planted now, though they will grow very slowly. I have only spinach, lettuce, turnip greens, and carrots growing now, since I was late removing my summer crops from my hoophouse to make room for sowing the winter crops.

The best resource for learning about unheated winter growing in a greenhouse is Eliot Coleman's "Winter Harvest Handbook." It can be frustrating ferreting out the information in a way that can be easily applied, but it's pretty much all there.