I ran across this site www.hgg.com (click here for site) with a ton of instructions on folding different envelopes. I was rather amazed at how many fun ways to fold an envelope. Not really sure if all of them can really go through the mail, but they are really fascinating and if you hand delivering it than I suppose it really doesn't matter.
Unfortunately, the instructions are hand drawn sketches and a bit crazy to follow, but I have really enjoyed trying to figure them out. Happy folding!