I love the Marshmallow Challenge! Taken from the 'official' website:
The task is simple: in eighteen minutes, teams must build the tallest free-standing structure out of 20 sticks of spaghetti, one yard of tape, one yard of string, and one marshmallow. The marshmallow needs to be on top.
So many great lessons learned. I used to do this with my physics students and one time I took this challenge to a 2nd grade class of a friend of mine to see if the younger students would indeed put the marshmallow on top sooner. Here is a blog post Ms. Cerda wrote about that day.