I use this lab with my 7th grade Earth Science class. We discuss density in both my oceans and my weather units. I use an excellent interactive (Google search Science Joy Wagon Density Interactive) to introduce the concept the day before, and then I have the kids do this lab the next day. They have a lot of fun trying to suspend the canisters.
The lab uses empty film canisters, and the students have to fill them with various objects to manipulate the density of the canister. I put out a variety of objects- pennies, paperclips, marbles, beads, brads...just use whatever you have available! They have to make one canister sink, one float, and one suspend. My kids use dishpans of water and it works perfectly.
When I do this lab, I tell them it's due by the end of class (our periods are about 40 minutes). I have an assignment for them after that is due the next day. Almost all kids finish the lab in class, no problem, and get a good head start on their homework (it keeps them busy after they're done with the lab!).