We decided to put up our Christmas tree early this year. It's one of those cheap artificial trees and we've had it for many years. (One year we had some mice make a nest in it.) After got it unfolded and fluffed up, without any prompting, Jonah started to "decorate" it. We aren't sure where he got the idea, because he was too young to do it last year.
He didn't bother with the actual Christmas tree decorations, though. Instead, he just found anything that was laying around on the floor and put it on the tree. Items included: half a plastic Easter egg, Shanna's scrunchy, a nerf football, a plastic candy cane, some dirty socks, a bib, a keychain, and a small beach ball.