Winter gardening in my unheated greenhouse

The title says what I do, but I have a couple of questions.
The first question is what to plant. I have two varieties of lettuce that have the reputation of doing well for winter (outdoor) gardens: Winter Density, and North Pole. I also planted Winter Bloomsdale spinach. I have one kind of broccoli in the ground. Cilantro is another that does very well in winter that I am planting both inside and outside the greenhouse. Is there anything else you think might do well?

The second question is the real motivation for this post. On the coldest nights, when it gets well below freezing should I have my greenhouse fan going all night long or intermittently, 1/2 hour on and 1/2 hour off. It needs to be on some because without a fan the coldest air will settle closest to the ground and will attack my plants, but having the fan on all the time makes the inside air hit the plastic more which will exchange the heat between inside and outside faster which will result in a lower overall greenhouse temperature.

I live in the Willamette Valley of Oregon. Our low temperatures get into the high teens for a week or two most years. Every few years we get below 10 and once in the last 35 years it got to a couple below 0 at my house. Through the winter we get very few sunny days, though a week or two at a time is common. The good news is that when it is cloudy it is mostly rain rather than snow and so it keeps the temperatures over freezing.

It's hard to give you advice without knowing your climate and the temperatures expected inside your greenhouse, so I'll just tell about my unheated greenhouse.

I live in a climate with a cold but mostly sunny and dry winter, minimums 0F to -10F and 6 weeks of pond skating, but it's only 34N so our winter days have fairly long hours of sunlight. According to timeanddate.com we have less than 10 hours of daylight from early Dec to early Feb, but in my experience the plants listed below never stop growing, and I think it's because the climate is so sunny. We heat our houses by attaching a greenhouse in October or November and removing it in April or May, so we don't heat the greenhouses, they heat our houses. The school I work at has been doing this for over 20 years, and I've been doing it at my new house now too. I grow vegetables and flowers in my greenhouse all winter, some varieties that are traditional in this region for greenhouse production, several that I tried based on Eliot Coleman's books, and some that I happened to try and they worked.

Inside my greenhouse certainly goes below freezing for dozens of nights, but leafy greens and a few others continue to do fine and grow through the winter. I've never used a fan in the greenhouse. Clothing on the line in my greenhouse often is frozen stiff in the morning in the winter, and some of the plants are even frozen stiff but revive as the day warms up.

These plants stay fine through the winter in my greenhouse over about ten years of my growing things in it:

Kale, any kind I've ever tried.
Arugula (British English rocket), both the annual white-flowered kind and the perennial yellow-flowered kind.
Mustard greens.
Spinach.
Lettuce, any kind I've ever tried.
Claytonia.
Dracocephalum moldavica, a delicious local lemon-scented herb that goes wonderfully with claytonia.
Local chinese cabbage or mustard greens called "salat" though it must be cooked.
Chard.
Dill.
Parsley.
Cilantro (coriander) but maybe not in the coldest months, I'm not sure.
Edible chrysanthemum, shungiku, a local vegetable/flower.
Carrots -- For winter I follow Coleman's suggestion and plant "Napoli" in September to harvest "candy carrots" in February.

Calendula blooms through the winter. Bachelor's buttons seeded in September bloom profusely in the greenhouse in March when there hasn't been any greenery outdoors for months. Perennial flowers that aren't quite hardy enough for outdoors in our climate thrive and boom in the greenhouse.

Perennial herbs that might not be perennial outdoors in our climate do great in the greenhouse. So far I've got: oregano, thyme, Indian "pudina" mint, fennel. I've got local chives and garlic chives in the greenhouse and can harvest green leaves from one or the other all winter, whereas outside they die back for the winter. Last year for the first time I had rosemary and I was amazed, the rosemary I left in the greenhouse in a pot survived despite dozens of nights below freezing.

So below-freezing temperatures don't both the plants in my greenhouse and I don't need a fan for that, but I would consider a fan for the hot days of spring when it gets roasting hot in the greenhouse but is still too cold outdoors to consider removing the glazing yet. I get a few roasting hot days every February, and then more and more in March and April until I remove the glazing entirely for the summer.

You might enjoy both Eliot Coleman books. The earlier one Four-Season Harvest inspired me to try growing a lot more different things, is more oriented toward home gardening, and has whole chapters of prose I enjoy rereading. The later one The Winter Harvest Handbook has lots of gorgeous color photos and builds on many more years of his experience, but I felt it was more oriented toward commercial gardening. I love them both and keep coming back to them and rereading them.

The roads to our region close for the winter every year for several months, and then we get no non-local fruit or vegetables in the market, so we depend on stored food all winter. I find that eating even a little bit of fresh vegetables from my greenhouse all winter makes a huge improvement to how I feel. Also, I love sitting in the greenery and watching the progress of seedlings or flowering in the winter, when there is no greenery outdoors at all.

Thank you for your response. I knew shortly after I posted that I should have said where I live. I edited my post to include the following:

I live in the Willamette Valley of Oregon. Our low temperatures get into the high teens for a week or two most years. Every few years we get below 10 and once in the last 35 years it got to a couple below 0 at my house. Through the winter we get very few sunny days, though a week or two at a time is common. The good news is that when it is cloudy it is mostly rain rather than snow and so it keeps the temperatures over freezing.

The main reason for having a fan in the greenhouse is to prevent disease. I asked a professional nurseryman friend how much he has his fans going in the winter and his answer was 24/7... but his houses are insulated and heated some.

Another problem I have is that the sun does not hit my greenhouse except for a few hours a day because of trees and buildings around it (I live in the city). My greenhouse is 24x8.

With a zone 8 west coast winter.
I think that you can grow pretty much any cool season leafy greens.
Cabbage/Kale family
Spinach/Chard family
Dandelion/Lettuce family
Onion/Garlic family herbs
Mint/Thyme family herbs
Dill/Carrot family herbs (some)
I would not do any fruiting plants (peppers, melons, etc)

The plants are going to need fresh air/carbon dioxide.
So some type of ventilation or maybe, add worm composting to add some carbon.

While I can pretty much guarantee that plants will not die due to cold, don't expect to see much growth.  

Elliot Coleman uses unheated greenhouses in Maine. He does not just use the greenhouse for cover: he also lays one, two, or three layers of plastic across his greenhouse  growing beds.

I have used this method in my own unheated greenhouse, and it works as long as the weather behaves itself.  Lately we have been getting VERY cold weather here in Kansas  in the early Fall and during what is supposed to be our spring, and so for  Christmas this year I am getting myself  a "Big buddy" LP space heater. I could not really use the greenhouse this Fall as the weather has been just too cold, but I am hoping to be able to use the greenhouse properly this spring!