I too am in Zone 5, in Prince Edward Island, Canada. I have an unheated 8 x 8 greenhouse (built it myself last year out of old windows), with a Suntuff roof. Last fall I extended the growing season until late October (I didn't really have much stuff growing in it because I just finished building it in the summer.) My biggest problem this summer has been to keep it well ventilated so the place doesn't overheat!
Right now I have melons, tomatoes, late beans, a squash, peppers, and even a pineapple top growing. I have been growing mesculin(lettuces) mix since the spring in two small plastic trays. It has been cut (and eatern) 5 times already and still growing well. I plan to plant a couple more trays of mesculin mix soon, and expect that I won't need to purchase lettuces until November. Spinach and other greens would grow well too. I like growing stuff in moveable trays, That way I can just harvest beside my kitchen sink and return the growing tray to the greenhouse.