Michigan State University is starting to promote high-tunnel or hoop houses as a way to extend the growing season here in MI. These are un-heated structures that are simple and easy to construct that is basically a series of semi-circles (or hoops) with plastic covering it. There are may cool weather crops that can be grown almost through the winter here is S.E. MI and can be long enough for a quantity crops to sell.
Another option would be a small greenhouse like the one described at DIY Greenhouse This would be a smaller structure for starting plants in the early spring and a few things for personal consumption.