I'm sure I've mentioned this numerous times, but we regularly pilfer cardboard from Costco for our art and craft projects. It's like giving the cardboard an extra day to bask in the sun before sending it back to the recycling.
This year we cut out some Christmas trees and let the kids paint them green with watered down food colouring.
Then they dragged them outside and stuck a load of Christmas stickers from Oriental Trading all over them. Pretty simple, but it kept them entertained for a long time, for the price of a bag of stickers and we got good decoration acreage out of it too.
I've put the left over stickers in a box next to the trees in the living room and every now and again the kids go back to it and add a few more stickers when they feel like it.