One of the best ways to use leftover egg whites is to make meringues.
4 oz egg whites
1 cup sugar
1/2 teaspoon lemon juice (or white vinegar)
Preheat oven to 220F.
Beat egg whites on high speed. Gradually add sugar, 2 tablespoons at a time. Beat well after each addition. Add lemon juice. Keep beating until stiff peak. Once you add color (if using) beat one more minute. Pipe meringue through a pastry bag. Pipe small mounds onto baking sheet lined with parchment paper.
Bake 1-1/2 hr. Leave them in the oven after it is turned off to continue drying without browning.
Here I used double icing bag to have two colored cookies.
You can make meringue sandwich cookie by filling two cookies with jam or nutella.
Or make a pie shell and fill it out with your favorite fruits and whipping or custard cream.
Meringue cookies are also beautiful cake decoration.