I thought about getting a set of density blocks for a demonstration but in the end I decided to use computer simulations instead. Both ExploreLearning and Adaptive Curriculum have several great simulations covering density and buoyancy. PhET has one for buoyancy and another for density that are free, as is the Floating Lab.
You could also read Archimedes and the Door of Science by Jeanne Bendick along with the first chapter of this book. The details of what happened when Archimedes figured out the principle named after him are in it as well as Fleisher's book.