Hi Molly;
Leftover mashed potatoes reheat really well in the microwave, just fluff them up a bit and reheat on high, time depending on how much you are heating, fluff again after heating, almost as good as when they're first mashed.
Here is a recipe using mashed potatoes that could be done with leftovers, I would reheat them first.
Shepherd's Pie

--------------------------------------------------------------------------------

Ingredients
5 cups cooked, chopped roast beef or pot roast (about 1/2 inch pieces, or smaller)
1 cup onions, chopped
3 cups thinly sliced carrots
1 cup celery, chopped
2 cups brown gravy (approximate)
1 beef bouillon cubes or equivalent base
3 teaspoons parsley flakes
1/2 teaspoon sage-no more
3 large garlic powder
1 teaspoon salt
Pepper to taste
margarine
mashed potatoes
cheddar cheese, grated
Saute onions, celery and carrots in margarine in large skillet just until tender. Combine with roast beef, gravy and spices and simmer slowly for about 20 minutes. Be sure to remove bay leaves. Mixture should be thicker than stew but neither too dry or too runny. Add more gravy if necessary. Put approximately 3/4 cup hot beef mixture into small, round casserole dish. Cover to edges with mashed potatoes, smoothing with knife. Mark with tines of fork. Sprinkle middle with approximately 2 tablespoons grated cheddar. Place under broiler until lightly browned or bake in 350 degrees oven for 15 minutes. Garnish with fresh parsley.

Serves: 10