Somerset Studios used to feature "Mail Art" at the front of their mag each time it was published, not sure if they still do it as I don't buy the mag these days. It was really good to see the different ways people used to decorate them - so you could stamp on them or cut pictures out and decorate them that way, a kind of collage. Maybe you can think of another way?
Go on, brighten someone's day and make them a card and decorate the envelope - I know it makes me smile when one pops through my letterbox.
Two samples of envelopes from Somerset Studio