This was always a favorite activity when I was in the Girl Scouts. We used to cut up calendars and magazines and make gorgeous mail. But one thing the Girl Scouts taught me, is the USPS is fussy about what they consider to be standard sized. SO first I made a template out of a pre-existing envelope and cardboard.
And finally, my vehicle has been targeted recently by several take out Chinese restaurants. SO I took advantage of the free menus.
Hopefully this inspires you to try something different, maybe re-purpose some materials, and send some whimsy in your everyday letters!