Our kids are also grown & gone, so I also cook for two. Meatloaf one night is meatloaf sandwiches subsequent meals. Get plenty of variety from a leftover turkey - sanwiches, casseroles with brocolli & mushroom soup. Whenever steaks are on sale we load up and we grill lots of extra, leaving them a bit rare. They are then cut into about 1/2 to 3/4 lb portions, freezer wrap them, and freeze them. They get thawed and thin-sliced to go into a stir-fry, atop a salad, or cubed into soups. Extra pasta usually goes into a stir-fry.
Mostly, though, a dinner with leftovers becomes lunch for me for the next few days, as I work less than a mile from home. Mrs. Deubster works too far away to return at lunch, so she eats those 8 - 9 oz. frozen diet meal things at her job (that would be an appetizer for me).