How do I decorate a plastic mailbox for Christmas?

How about a big plastic bow around mail box .

A big red bow,pine garland & battery operated lights. use 2 ribbons 1 white 1 red and twist them around base/pole for candy cane look.

You can use command hooks and hang plastic ornaments at various lengths from the bottom of the mailbox.

You could hang some ornaments on it or you could add some stickers to it. You could add a garland around it.

Buy red and white plastic table cloths from dollar store. Cut them into strips. Wind these strips around your mail box and staple them in place. Put a garland swag over the top of the mail box. Then make a big red bow to put in the center of the swag. Use zip ties or wire to hold garland and bow in place.

Take a look here:

https://www.pinterest.com/PeggyAveritte/christmas-mailbox-decorations/