Once you got an idea in mind, pool together household objects and toys — dominos, aluminum cans, cups, cd cases, and marbles all work well — and brainstorm on how to make your Goldberg machine do its task. You can draw it out, or just build things on the fly and see how they could work together.
This PBS Kids site has some great ideas you can use for building such contraptions, with several children's comments on what they built and made. You can also see what these 6th-8th graders did for inspiration. It's sure to be a fun and stimulating activity that you can scale to your desire, whether it be a multi-week summer activity or something you do in an evening. Try a Rube Goldberg out, and promote the fun of science. We'll leave you with what has to be one of the most complex Rube Goldberg's ever filmed — proving that some kids never really grow up. (Images: 1. Flickr member eedrummer. Licensed for use under Creative Commons 2. Seize the Play 3. Chris Perez)