For the final two weeks of 6th grade science I decided to ask the students to build Rube Goldberg machines. Most of the project is explained in the video above, but in general the students were put into groups and asked to build devices that threw away a piece of trash and made use of at least 1 pulley and 1 lever. They spent about 7 class days on these projects and the results were encouraging. Even more encouraging were the comments at the end of the final exam that noted the Rube Goldberg machines as the 'favorite activity of the year'. I even heard from other teachers that the 6th graders were discussing the project outside of class, which was nice to hear.
From a teaching perspective, this project might have also been the easiest/best thing I did all year. Prep time was minimal after the initial gathering of supplies and project assignment, and grading was even easier than that because all the groups did such a good job overall.