Well, I've put this 2ft tall motion sensor toy behind the tree. It greets a loud "Merry Christmas!" and then plays a couple of carols while expanding and shrinking in size. As you can see, the tree is left alone. I hope it lasts for the season!
UPDATE
The motion sensor toy worked too well. The cats have not touched the tree but that thing kept on waking me up in the middle of the night or early morning. I suspect the kitties decided it also made a good alarm clock for their mommy.
I removed it and gave it to my inlaws (they have no cats but 2 very curious granchildren).