In years past I have always cut the ties of Dad's who come in to the room (having given them old ties to wear). This year I walked around the building taking pictures of Dad's hands and writing down their favorite primary song. It's really easy and you can show the pictures on a laptop or tablet, or print them out. The idea is to have the kids guess who's hands they are, and see if they can recognize their own Dad's hands. I also took pictures of the Bishop and his counselors.
Please see my other Father's Day posts!