This is a project I did with a friend just before Christmas that I was able to get my grandson involved in.  The idea is to use a CR2032 coin cell to power LEDs as it rolls down a ramp.  The project is constructed from foam board cut out with a hobbyist knife and copper tape used to make the connections with the coin cell.  It is best understood by seeing it in action....

My grandson thought this was the greatest and even suggested an improvement for next year:  Put 25 LEDs on the tree and then use jumpers to add a new LED everyday.  Every day a new LED would be added and the tree would count up to Christmas.

 

It doesn't get much simpler than this.  We entered it in the Hackaday single coin cell competition.  If you have additional interest in the construction and other ideas we looked at then see it here:  https://hackaday.io/project/28793-gravity-assisted-blinking-coin-cell-christmas-tree