There is no easier costume than a paper bag mask. Grab a paper bag and cut out holes for your eyes then grab a few markers and let your imagination go wild. They are so simple for kids to make and to wear.
Interdependence Tree Project in Huntsville, Alabama by The Dream Rocket Project. Trunk-wrapping for live trees for a long period isn't good them but there's a dead tree at Talkin Tarn if you've wrapping ambitions.