For this project, I used a grapevine garland that I purchased at Davis Wholesale Florist:
The garland wrapped nicely around my mailbox. I didn't have to secure it in any way.
Then I added branches of leaves that had a nice mix of autumn colors. I purchased floral water tubes at the Mt Brook Western and placed them on the ends of each branch. The steady drink of water will keep them from drying out as quickly and are easily disguised within the greenery.
This is a quick project that adds an unusual touch to your mailbox. I am going to try the grapevine as a base for my Christmas greenery next month.
Addendum: Even with the floral water tubes, the leaves dried out after a couple of days. I found silk autumn leaves that looked amazingly real at Hobby Lobby. I used them instead, and I actually liked them as much as the real ones.
Either way you choose to go, it's a really fun project.