When my children were small, my DH would go and pick them up from nursery school and take them to his parents house and eat dinner there. He told them that mom had a faculty meeting and would be late coming home. I would hurry home and get all the decos out and put the tree up. Then I would call my DH and tell him I was finished. He would bring the kids home, unlock the door, and when they entered the tree would be lit and family room decorated. Santa's elves would leave a note saying they would be back with Santa on Christmas eve.The kids would be jumping up and down. Then I would come in from the garage looking like I just got home from school and act so surprised!
This tradition went on for years. Then one year when they were in High School, and I was just too tired to do it all myself, the tree was left out on the patio with a note saying that the elves were running late and we would have to decorate ourselves. This started the new tradition of all of us decorating it together.