This is a fun experiment I like to use with my kids. I use it to teach the Scientific Method, but it could be used for anything really. I start by giving my kids a question. From there they have to create a hypothesis. The students "create" a list of the materials needed and procedures for the experiment. I really have a good idea on how the experiment will work out, I just guide them. We then preform the experiment and present our data in a double bar graph.
I would normally break this activity up into three 45 min blocks.