How about a Clematis Vine? There are alot to choose from.

I was going to say clematis, too... that would be my first pick. Some of them require a bit of pruning back when they get too large but it's not a high-maintenance chore, usually just once a year. They always look charming on a mailbox although the bloom season isn't that long.

There are lots of things you could pick from; just make sure it isn't something that draws a lot of bees in case your mail carrier is allergic. Also, some of the perennial vines are aggressive and get huge, so check into their growth habits before you buy. If you want an annual, black eyed Susan vine, Thunbergia alata, is a possibility (I don't *think* it attracts bees, but someone correct me if that's wrong -- I've never seen them on mine, anyway) -- very bright and cheerful and blooms for a long time.

Have fun picking something -- lots to chose from!

In my old house, I did a clematis. In my new house, we have community boxes so no more clematis :( I also had a fountain grass and daylilies around the box.

Here's a pic of my old mailbox:

Meigzella, your "old" mailbox is beautiful. What type of trellis did you use for your clematis? I planted some around my mailbox last year, but they are not nearly as lovely as the one in your picture!