I’ve always loved the compressed air rocket. I bought my son one of the pre-made ones at a science museum and it broke on first flight, which was pretty frustrating. Had I known about this MAKE video, however, I would have built my son one from scratch.
You technically don’t need more that 25 cents worth of hardware for this thing, but getting some of these parts might be cost a bit more. Here is this how-to PDF and here’s the original article.