Intro: SS YM - Lego Tower
If you have a big tub of lego this is a really fun group activity for all ages.
For my young makers group (14 year olds) we challenged them to a race to make a free standing lego structure that started from the ground and reached the 11 foot ceiling.
For my 9/10/11 year olds the challenge was just to make a connected lego structure that reached the ceiling. You could hop on a chair and help support it. Being Sunday School you also had to have a sword (aka hockey stick) with you at all times to protect you from being attacked by the Ammonites. The lego structure represented the wall of Jerusalem as supported by Nehemiah ... well too much information :)
I think you learn a fair amount about engineering and structures doing this.