Earth Balance buttery spread is a great butter substitute for baking. I've even made caramel with it, that was indistinguishable to non-vegans from the real thing.

Cream is a little harder - there are vegan cream products, but I haven't found one that isn't weird. Maybe you can just save out a portion of, say, the mashed potatoes, before you put cream in them?

If butter and cream are the only issues, you can probably subsitute palm shortening and nut milks (like a cashew cream). Or use Earth Balance instead of butter--it tastes very similar and works the same in recipes. I made an apple pie crust with it a few years ago and it worked great. This Thanksgiving I'll be making baked tofu flavored with poultry seasoning.

Mashed potatoes made with vegetable stock and lots of roasted garlic are delicious. Roasted butternut squash and coconut milk soup is also a very tasty vegan dish....really you can use the coconut milk in place of cream in a lot of traditional dishes, the flavor is mild enough that people might not even notice and you wouldn't be feeding people that nasty additives and stabilizers that a lot of vegan versions of dairy items contain.

The red wine roux from Veganomicon is a great gravy substitute: http://recipes.sparkpeople.com/recipe-detail.asp?recipe=524157

I agree that butter/cream are fairly easy to replace. I also like Earth Balance--and soy milk or almond milk are fairly creamy, so they stand in relatively well for cream. I use flaxseed meal to replace 1 to 3 eggs in baked goods (just did it this morning with some muffins) and it works like a charm! Pumpkin stew in a giant pumpkin is beautiful, as are wild rice dishes. And the NY Times also had some good suggestions: http://www.nytimes.com/2002/11/20/dining/so-who-needs-a-bird-vegetarian-entrees-for-the-feast.html?pagewanted=all&src=pm

Here are several yummy sounding vegan options: http://www.theppk.com/2010/11/thanksgiving-in-an-hour/

Whole Foods also has some good ideas.

Earth Balance shortening, buttery sticks and spread are great vegan substitutes. For heavy cream, MimiCreme (found in an aspetic package) is excellent. They also make a whipped topping.

For recipe ideas, try Robin Robertson's blog -- there's a lot of Thanksgiving on there right now! http://veganplanet.blogspot.com/

I use Thanksgiving to indulge in vegan "meat" entrees -- the seasonal Field Roast en Croute is a fantastic holiday splurge!

I know it's already been mentioned, but Earth Balance is amazing I'm not even vegetarian anymore and I still use it for everything.

Also, Silk has a great soy creamer (intended for coffee, so make sure to get plain, not vanilla!) which is a great substitute for those things.

And this is *amazing*! http://blog.fatfreevegan.com/2006/11/best-vegan-green-bean-casserole.html

Be sure to check out her Vegan Thanksgiving and Holiday sections, too. http://blog.fatfreevegan.com/tag/thanksgiving
http://blog.fatfreevegan.com/tag/holidays

This was always my favorite vegan Thanksgiving entree - it comes out very thick and hearty, almost not quite a stew:

http://www.theveggietable.com/recipes/mushroombarleystew.html

Veganimicon has a delicious southwestern corn pudding made with coconut milk. My husband is lactose-intolerant, so butter/cream-free dishes are appreciated, even though he's an omnivore. It was corn, diced red peppers, and the coconut milk with seasonings, and baked. The recipe is Mexican-spiced, but you could easily adapt to go with the rest of your menu.

Veganomicon's Chickpea Cutlets!!!!!!! Holy crap these are delicious (and obviously, vegan)!

I've made them the last two years for Thanksgiving.. along with any other time I wanted them. :) Last year I browned the cutlets, baked them on top of stuffing, occasionally basting them with a mustard-maple syrup mixture. SO GOOD.

Arielle: one thing that helps me is when using any cream substitute (in a dish that will be cooked) I add a bit of agar flakes or powder, I prefer the flakes, and that approximates heavy cream very well in sweet OR savory dishes.

I think it's really wonderful that you're willing to accommodate your vegan guest. If you do want keep a few of your butter/cream favorites on your table you might consider adding a balsamic or maple-glazed roast root vegetable dish with fried sage leaves (really nice touch but a last minute garnish so be forewarned) along with quinoa as a purely vegan and relatively high protein main dish.

I am a vegan! and in things like mashed potatoes I use vegan butter and tofutti sour cream. My non vegan friends and family have no idea and it provides all the rich creaminess you are used too. I know vegan cooking seems daunting at first, but I promise it isn't. Field roast makes a delicious celebration roast that makes a wonderful main dish- it's available at whole foods.

here are some blogs that have easy accessible recipes too:

http://www.meettheshannons.net/
http://kblog.lunchboxbunch.com/2011/10/four-ingredient-vegan-pumpkin-pie.html

Good luck! And remember not too stress out!

i think its nice to take a guest's dietary restrictions into consideration for a least a dish or 2. being a fairly recent vegetarian i typically bring a dish that i know will at least fill me up in case there is nothing else for me to eat

I've got a seasonal fruit salad recipe that's great for Thanksgiving. I used honey, but for strict vegans, agave syrup would be a fine substitute!
http://operagirlcooks.com/2011/11/17/thanksgiving-fruit-salad-recipe/

I do a stuffed pumpkin except I stuff it with a nut loaf recipe with fresh sage and other herbs. I have made it vegetarian and vegan and it always turns out!

mashed potatoes: no need for fancy creams, just use earth balance and soymilk. Delish.

sweet potato casserole- again, sub out butter for earth balance, and top with Dandies vegan marshmallows.

Steamed greens. Already vegan.

Green bean casserole- use the mushroom gravy that you're already making instead of cream of mushroom soup. You could make it "creamier" with soymilk or soy cream.

stuffed acorn squash- stuffed with stuffing or a seasoned wild rice

Field roast really does have amazing vegan main dishes. Delicious!

I think you'll be surprised at how easy and delicious these sides can be. I have been vegetarian for over 4 years, and last year was my first vegan t-giving. I was shocked that everything was still so rich and delicious, and so easy to make.

Post punk kitchen has a great pumpkin cheesecake recipe. Vegan cookies are easy to make, and in my experience pumpkin pies take a little more finesse to veganize properly and still taste good. But it can be done! You could also be super generous and make these vegan pumpkin cinnamon rolls for your guest. http://donteatoffthesidewalk.com/2007/11/08/veganmofo-i-hope-you-have-your-fat-pants-ready/ They are TO DIE FOR.

Instead of of subbing with the suggestions noted, I'd seek out some truly vegan recipes and include them.

Earth balance and soymilk taste like Earth balance and soymilk, not at all what I want to taste in my Thanksgiving mashed potatoes.

How about wild rice and cranberry salad, mushroom gravy, kale salad with pomegranite seeds, and corn pudding. All delicous, without the off tastes off substitutes.

I use a mix of almond milk and coconut milk when subbing for dairy. Coconut cream can also be whipped and used as a dessert topping (refrigerate a can of coconut milk, skim off the cream, and save the leftover liquid for soup).

And use whipped Earth Balance, not the regular kind (and especially not in stick form - it tastes different). It spreads much more easily.

My butter-addicted French mother happily eats my dairy-free cooking and if she can't taste the difference...

Arielle! We host a Friendsgiving and accomodate quite a few of dietary needs, here are a couple of ideas:

Vegan mashed taters: I usually make my non-vegan version with mostly butter, sometimes a little cream, and goat cheese. I've found that subbing in the vegan smart balance (careful some have whey protein added to them), or margarine, with vegan sour cream, chives/green onion, make a really delicious version that vegans and non-vegans will enjoy! Plus, it's really simple to cook big batch of potatoes, make one non-vegan batch and one vegan.

One of my favorite soups in the world is Curry Carrot Ginger, very much a fall/harvest meal, and totally appropriate for Thanksgiving. I believe that thekitchn recently posted a version of the soup-- and this is my version, totally vegan!
This makes what I'd call a medium amount of soup, probably enough for 6?
3 Shallots, half of an onion, or something else aromatic (leeks would probably work too). I use shallots because I find their flavor more subtle.
olive oil OR butter (oil makes this vegan, easy substitution)
2lbs carrots
2 large parsnips (adding parsnips seems to make a less sweet soup-- which I like)
Curry - I think most any will do, I like the richness of good ole standard yellowy curry
Cayenne (optional)
Herbs/spices (i round up whatever is in the house, usually BAY LEAF, thyme, basil, sage -- tumeric, coriander)
Stock- if you're making a meat version, chicken stock is delicious, homemade veggie stock is obviously the tastiest, and a canned/boxed/prepped stock/broth is also just dandy! (For emergencies, when we don't have stock in the freezer, we keep a jar of better than bouillon in the fridge, very salty but pretty flavorful regardless)
can o coconut milk

Cut your shallots or onion-y product
Roughly chop carrots and parsnips (I aim for the same size)
Heat a stock pot on med/high heat, add your butter or oil, sautee the onion-y product, once warmed add in your curry, let that heat up with your onions, I like adding cayenne (a couple shakes) because I think that curry and spicy work for me, ..get them sauteeing-- add in the bayleaf, thyme, herbs you're using, once translucent/beginning to brown, I add in the carrots/parsnips, let them cook a bit, probably between 5-10 minutes, depends how impatient/distracted I am. Add in the broth, bring up to boil, turn down the heat, partially cover, and simmer for about 20-30 minutes.. take off heat, retrieve your thyme/herb sticks, and bay leaves, blend the crap out of it, I use an immersion blender, but you can also do it in batched in a regular blender, or use a food mill. return to stove top, make hot, but do not boil, add your coconut milk.. and viola!

there are a number of tabouli/quinoa dishes that we have cycled in over the years.

I have made stuffed pumpkin several times - once with a savory filling of rice and lentils and once with sweet rice, apple and dried fruit. I've also had one filled with curried chickpeas. Delicious!

the best (only???) cooking trick that actually makes food better, not just faster, that i learned from my time with food not bombs? olive oil in mashed potatoes (or other roots) instead of cream or butter. it makes it creamy and tastes delicious! and people can still put their own earth balance or butter on top.

also for a main course, i like to make seitan pot roast (http://veganplanet.blogspot.com/2008/03/slow-cooker-seitan-pot-roast.html) which is very little work, and cook it halfway. then i put it (the whole slow cooker insert!) in the oven at 350, and it gets all puffy and delicious. i take it out after about 45 minutes, and stuff it, either with the veggies that have been roasting with it or stuffing. i brush it with oil and put it back in the oven for another half hour or so and it gets so browned and delicious!

also, brussels sprouts, obvs.

I've been working on vegan Thanksgiving ideas for weeks now. I'm almost - but not quite - tired of Thanksgiving food! I love the suggestions the commenters are making here.