Added flipped Christmas tree into bright area at left background.
Adjusted color balance and contrast with curves.
Cloned over some of the bright spots on camera left sides of faces.
Brightened the trees and ornaments with Hue/Saturation.

forum discussion: http://forums.dpreview.com/forums/read.asp?forum=1006&message=26320710&changemode=1
original image by Alan Joyce: