My go-to icing for making a vanilla sheet cake is a simple vanilla buttercream. I also love to use icing like cream cheese frosting, lemon frosting, or a delicious chocolate ganache. You can use just about any type of icing on this cake and it will be perfect since this is such a versatile cake. If you don't plan on decorating with a piping tip, you can also simply spread any generic store-bought icing over the cake. It really is up to you and your preferences.