what about a Brussell sprout salad? With apples and blue cheese and a nice vinaigrette?

I am making Pecan Crusted Seitan from Alicia Silverstone's cookbook The Kind Diet. You marinate overnight, and then fry on the stove right before serving, no oven needed. Contact me if you would like the recipe. Happy Thanksgiving!

Not really traditional Thanksgiving, but if you have access to a pasta roller, you could make fresh pasta; or make gnocchi w/out a roller (you can use water or an egg substitute if they don't eat eggs).
Here are a few of my recipes:

Ricotta Gnocchi in Tomato-Basil Sauce
Fresh 4-Cheese Ravioli w/ Mushroom Sauce Swap out cheese for spiced pumpkin filling, and the mushroom sauce for a sage brown butter sauce and you get a festive fall dish.
Fresh Fettucine w/ Pesto, Broccoli, and Mushrooms

Do they eat fish? If so, my Sole Meuniere is a great and simple dish.

For a more festive dish, you could make a butternut squash risotto (just make sure you use vegetable stock).

Mushroom-cranberry pasta in basil cream sauce: http://emuisemo.com/?p=652

Garnish it pretty, and you're good to go. If you really want "fancy," use a greater variety of mushrooms (since you'd have to scale it up vastly from my two-serving level anyway).

Vegetarian Times has a great wild mushroom shepherd's pie in its most recent edition. It also has other suggestions for vegetarian dishes for the holiday - but only in the print version.

See other versions in this link: http://www.vegetariantimes.com/recipes/search_recipes/?keywords=shepherd's pie

I just saw that it was stove top - I am sorry. Slow with thoughts of tomorrow's feast. Vegetarian Times has a number of ideas for stovetop cooking. One thanksgiving I made a sweet potato stew, it was delicious.

Mushroom and Seitan Stroganoff

2 tbsp. butter
9 oz. seitan, cut into 1/8-inch (.25 cm) slices
1 onion, chopped
1/2 tbsp garlic ?
1 lb. mushrooms, cut into quarters
1/2 cup white wine
2 tbsp. flour
1/2 cup vegetable broth
1/2 cup sour cream
2 tbsp. chopped fresh parsley
salt, pepper and paprika to taste
16oz pkg wide egg noodles
Heat 1 tbsp. of the butter in a large frying pan, add the seitan slices, and fry over medium heat, turning the slices over as they become slightly crisp and golden. Remove from the pan, leaving as much as the butter behind as possible, and set aside.
Add the remaining 1 tbsp. of butter to the frying pan. Dump in the mushrooms and onions and cook, stirring, over medium heat for 8 to 10 minutes, until the mushrooms have released all their liquid and it has evaporated. Add the wine and cook until it is slightly reduced - about 3 minutes.
Cook noodles per instructions, drain and set aside. Keep warm if done before the stroganoff
Sprinkle the flour over the mushrooms, stir, then pour in the broth and bring the mixture to a boil. Return the seitan slices to the pan, lower the heat and let simmer for a couple of minutes. Add the sour cream, and parsley, and season to taste with salt and pepper. Heat through, stir in egg noodles.
Makes 3 servings.

Go look at Heidi's recipes at http://www.101cookbooks.com

Tons of great options.

Mmmmm along the same lines as the stroganoff recipe above, I *LOVE* the smitten kitchen mushroom bourguinon!

http://smittenkitchen.com/2009/01/mushroom-bourguignon/

There's a super great vegetarian shepherd's pie recipe on epicurious, it's supposed to be stuck under the broiler for a couple minutes at the end, but you could totally skip that.

http://www.epicurious.com/recipes/food/views/Vegetarian-Shepherds-Pie-355994

Second the mushroom bourguignon from Smitten Kitchen! Even if you can't find pearl onions, it's 100% worth doing, and I find it to be a fuss-free recipe that goes together quickly. Oh, yum.

I'm vegetarian and for my main dish I'm doing a lovely risotto. I love risotto because you can add just about anything to it to make it heartier and its such great comfort food, which is what everyone wants on thanksgiving!

A winter squash or pumpkin tagine is festive and seasonal.

I agree with the risotto crowd! I'm making this one, but subbing veg broth for chicken, and leaving out the parm: http://www.epicurious.com/recipes/food/reviews/Roasted-Butternut-Squash-Risotto-105725

I'd go with Isa Chandra's chickpea cutlets -- you could make them beforehand, then form them and fry them stovetop at your destination. Enjoy!

Lentil rice pilaf! Of course, I say that whenever anyone asks what they should make for anything. :)

Thank you so much for all of your advice! It's going to be hard to choose, but I'd better soon.

I'm making israeli couscous with caramelized onions and mini heirloom tomatoes: http://5secondrule.typepad.com/my_weblog/2010/09/recipe-for-caramelized-onions-with-israeli-couscous-and-cherry-tomatoes.html. The colors are beautiful and it tastes great!

You could bake a puff pastry shell (vol-au-vent) in advance, then fill with a great veggie filling you make/heat on the stovetop. Here's a picture of some festive-looking ones: http://www.guardian.co.uk/lifeandstyle/2008/dec/06/vegetarion-recipes-christmas-nibbles

Those chickpea cutlets are to die for. Serve them up with a mushroom/thyme/white wine gravy on a nice serving platter, garnish with baby arugula and lemon wedges. Delicious.