This semester I took a student-taught course called "Rube Goldberg Machines: Theory and Practice". We made Rube Goldberg machines. For those of you who don't know what that is,
you can find some more info here. Basically, It uses very complicated machines to accomplish simple tasks. For example, in order to press a button, someone drops a marble down a ramp, which falls into a basket, pulling it down, which pulls a lever, which releases a softball down another ramp, which triggers something else, and so on for many, many steps.. eventually resulting in the button getting pressed. They are pretty fun to make, and watch.
It's pretty easy material wise, because it ususally invoves random found objects. If you design a machine in advance, and bring the supplies specific to that design, the kids can just assemble it. Though the machines can get pretty complicated, you can still get good results through something a 5 year old can put together, especially if they have a little guidance. Another method is to just bring a whole bunch of random objects, and have them design and create their own machine. That method is a little more complicated though.
What's fun is if you have each group create a machine, and then have group 1 trigger group 2, group 2 trigger group 3, group 3 trigger group 4, and group 4 accomplish some task. You could also just have the machines separate and have the winner be the machine that accoplishes the task the
slowest.