My school has a lovely tradition called "Grand Day." The Thursday before Good Friday, grandparents are invited to school with their grandcildren. There is a nice breakfast and fine arts performances for them. In the classroom, we design a lesson that the kids can do with their grandparents.
In my class, we do a variation of Bunnycopters. The kids can explain to their grandparents how we design experiments and control and manipulate variables. The grandparents really get into it, and the kids have a great time explaining experiments. I use bunnycopters because it is right before Easter.