I used them to make potato patties. I can't even tell you how good they were. Not healthy, mind you. But good. VERY good. If you've never made potato patties using leftover mashed potatoes, you are missing out on one of life's greatest pleasures.
I had three cups of mashed potatoes from Thanksgiving dinner.
I added two beaten eggs, one-half cup of self-rising flour (you could use 1/2 cup regular flour mixed with 1/2 teaspoon baking powder), pepper, and 1/3 cup of finely diced onions. I shaped this into patties and fried in 1/2 inch of oil.