Got a unit on circular motion? You may want to use an activity with a centrifuge, to show how it separates substances of different densities. Even if you’ve got a commercial centrifuge, how might you instead do a hands-on activity to show the same thing?
Try mixing red colored sugar in cooking oil in a syringe (10 ml). You can use the holiday colored sugar, or dye your own with food coloring. Attach string to the syringe (very securely!) and swing it around your head. This simulates a hematocrit test, which measures blood count, and the mixture looks a little bit like real blood. The sugar represents the packed cells, and the oil is the color of plasma.
Thanks to Karen Kalumuck of the Exploratorium for this idea. If you want a copy of her write-up of the activity, write me (stephanie at sciencegeekgirl dot com).
Another low-tech centrifuge is a salad spinner, which you can get at any thrift shop. Separating vegetables from water is not quite a density-driven process, as the vegetables are large and the water is driven to the outside because of its fluidity. Though, you will find that carrots end up on the outside and lettuce on the inside.