I wanted my students to understand density and buoyancy, and a fellow teacher clued me in to this website maintained by the University of Colorado that has a treasure trove of science simulations.  We developed an exploration worksheet and then we were up an running!  My students got to change the density, mass and volume of different blocks that they would drop into water.  The power of this simulation is how easily my students could manipulate different blocks to get feedback on what makes things float or sink almost instantaneously.  the density playground has many different ways to adjust variables and behaves as objects do in real life.  They could even change the density of the liquid that they drop their block in so they could see what would happen if you drop a block of ice in oil or rubbing alcohol.  Spoiler alert!  It sinks!