So this year my class is cruising along much faster than last year so I'd been thinking I'd toss in some floating and sinking to occupy a week or so of class. It's something that they have experience with, that is assuming they've been in a bathtub or swimming pool before.
When I saw that the PhET folk had created a floating and sinking simulation I was sold on the idea.
I did my usual which is to have the students work through some guided inquiry type questions. Seemed to work great for most, a few hiccups here and there. The questions could use some refinement, but in the end they picked up on the two main ideas. That is that floating objects displace their mass in water. Sinking objects displace their volume. That's good a start as far as I'm concerned.