According to Wikipedia, the giving of "May Day Baskets" anonymously on neighbors' doorsteps has faded in popularity since the late 20th century. Thank god we have people in the midwest that don't pay attention to trends that have been deemed outdated. May Day baskets are still alive and thriving in the centrally located city that I hail from, as the photo above from my sister this morning shows. Her two beautiful little tykes decided to step it up a little bit and hand out these miniture mailboxes, complete with letters and candy inside.
If you think this is the cutest idea ever, and you live somewhere where people actually have doorsteps to place these, and lawns to run and chase the giver on, you can find the white mailboxes at Target. As for the creative ideas for decorating them, my sister is one of a kind, so I'm not much help.
For more information about May Day...a celebration that has been around for centuries (pre-christian holiday in Europe in fact, which used to be the Celtic celebration of Beltane), go HERE.