This smoked brussels sprouts gratin can be completely assembled the day before and then cooked on Thanksgiving (http://www.marthastewart...). This maple roasted brussels sprouts and rutabaga dish can be made the day before and re-heated.
(http://www.wholeliving...) Virtually any cranberry relish can be made the day before. Here's a no-cook one
(http://www.marthastewart...)

Of course your mashed potatoes are best to make the day before. Heat them back up in your crockpot so you aren't taking up any stove or oven space.
Creamed spinach is great the second day.
Sweet potato and gruyere casserole is always a hit.

Mama Stamburg's cranberry relish from NPR fame: http://www.npr.org/templates...
Looks wild...tastes great.
But don't forget the option of the canned stuff with the ridges.