Round cookies can be used to make faces or jack o'lanterns. You can also use a cookie cutter if you prefer to make ghost cookies or other shapes.

Ghost
Combine icing sugar with a little water; spread over the baked cooled cookie. Make eyes with icing colored red, green or black. You could also use a little piece of licorice. 

Pumpkin
Add some orange food coloring to your icing sugar-water icing. Transfer to a piping bag and draw the features. You could also stick little orange candies into the icing to make prominent eyes. Add a line of green icing to make the stem.

Witch's hat
Add some black food coloring to your icing sugar-water icing. Transfer to a piping bag with a star tip with gold icing to decorate the hat, or choose mini candies. For the ribbon, use a contrasting color and a thin piping tip or a strip of licorice.