This is a lab that I developed for my seventh grade students. It takes a bit of set-up and planning but I love it. Students will measure multiple volumes of the same liquid and see using data that the density of a liquid does not depend on its volume. Four different liquids are measured on day one but each group will only work with one liquid.
On the second day of the lab I have students work together to compare densities of the liquids they used and then predict how to order them to create a four-layer density column. Students then test their ideas by layering the liquids in class. They really enjoy this part!
I do this in two 40 minute periods. Set up takes about an hour.