Tell them to suprise you, but leave an ornament or two for you to place.
My sister is coming home the same weekend and she wants to help with the tree. It's easier on everyone if I just go along. I love the way the tree looks when we get it up, but actually putting the ornaments on is an exercise in... well, imagine nine people all trying to decorate the same section of the tree; nobody wants to get on the ladder to reach the high parts; and of course the squabbles over the fact that we never have enough ornament hangers.
Still, maybe I'd miss it. I would miss the hiking around looking for the right tree.