We got the idea a year or so ago, when my two older boys took a "Rollercoaster" class at Discovery World. Using simple foam pipe insulation, tape and marbles, the boys were able to explore the rules of physics. And have fun!
In their class, the divided up into teams and competed to see which team could build the most complex, most successful rollercoaster. Points were given for the length of the "coaster" and the amount of time the marble stayed on the track, with extra points awarded for each successfully navigated loop-de-loop.
They enjoyed it so much that I ran out to Home Depot to purchase a bunch of pipe insulation. Since then, they've periodically pulled out the supplies and spent hours experimenting with angles and loops.
Cheap, easy, entertaining and educational -- my favorite kind of activity!