Print your business logo and your return address on the envelope to customize it for business purposes. You can do this using a printer or you can have it done at a print shop if you need it done in bulk.
Color areas of the envelope in marker to give the paper more variety in appearance. Leave yourself enough space to write a greeting on the envelope if you are using darker colors, or else your writing will not be visible.
Attach stickers or decals around the envelope that pertain to the theme of your card or message.
Write a greeting on your envelope in either a colored pen or marker that can be viewed over any coloring already on the paper.
Use glitter or any other decorative materials you wish to add to the envelope.