I'm not a big fan of doing "things" with leftovers. I like to make things that are eitiher eaten on the spot or are good a second time in their unaltered state -- turkey as turkey sandwiches, a roast beef as sandwiches or as cold roast beef with appropriate accompaniments.
But there are always exceptions -- especially when last minute cancelled guests mean there are lots of leftovers. Here are a few of my never fail tricks for using things up.
This is so old I'm not sure where it comes from. I've even made it from scratch, using deli turkey breast, for a buffet. It's easy.
Make a batch of white sauce (you need to make enough based on how much turkey you're using, but let's guess that you have enough for a 9x13" casserole). For that amount, I'd make 3 cups, so 6 tablespoons butter, melted in a large saucepan, then melded with 6 tablespoons of flour and heated, stirring, over medium heat, for a few minutes. You don't want raw flour. Stir in 3 cups of milk (2% is fine), gradually, and then season with salt, pepper, and freshly ground nutmeg (1/2 tsp). Mix in 2 cups of shredded white cheddar cheese.
While that's going on you can cook one pound of noodles. Long thin noodles (even angel hair) are traditional, but I often use shorter varieties; cook to just short of fully done and drain, then mix into the white sauce.
Cut the turkey (3 cups is ideal but 2-4 cups will work) into nice sized chunks and mix it in. As a variation, you can also add a head of broccoli, cut into pieces, and cooked to nearly done.
Put it all into the buttered (or sprayed) pan, sprinkle some more cheddar and/or grated Parmesan cheese on top (I like a mixture of both) and bake at 350 for about 40 minutes until bubbly and browned.
This works with many different kinds of protein leftovers -- turkey, chicken, pork, shrimp (does anyone ever have leftover shrimp?), and even beef. Just cut it into neat 1" chunks.
Melt 2 tablespoons of butter (or use vegetable oil) and cook one large onion, chopped, until translucent. Add one peeled green apple and one peeled pear (or two apples), thinly sliced, and cook for a few minutes to soften. Add 2 tablespoons minced garlic and 2 tablespoons minced ginger (from the jars -- use less if this will be too spicy for your family, but it really isn't). Mix. Add 1-2 tablespoons curry powder (use your favorite brand and level of spiciness) and 2 tablespoons of flour. Stir everything well and cook for a minute. Then mix in 2 cups of chicken broth (a 14-oz can will do), the juice of one lemon, and its grated rind. Stir everything gently but well and cook on a low flame for ten minutes until the fruit is soft and everything is thoroughly combined.
Now, add the cubed meat and stir in. Cook until the meat is heated through and serve over rice with chutney or other condiments (a mango salsa works well).
My favorite way to eat leftovers is to create a platter of cold (but not ice-cold) protein, served with something hot, a spicy relish, and a salad. It's easy, it presents the leftover at its best, and it provides a real meal. For instance:
Cold roast beef, a hot (fresh) baked potato, chutney, and a tossed green salad
Cold turkey breat, a hot baked sweet potato, cranberry sauce, and an argula, tomato, and avocado salad
Cold roast pork, sliced potatoes baked with cream and gruyere, fig jam, and an orange and fennel salad
You can make up your own.