A great lab for demonstrating the law of conservation of matter. Students use Lego blocks (or you can substitute gummy bears or some other manipulative) to build chemical equations. Students take the mass of the parts of the equations and then see how the mass is unchanged in the reactants and products.
This version has two versions: one with pre-filled colors, and the other with the colors left blank so you can adapt it to fit with the Legos you already have.
The second part of the lab includes alka-seltzer, water and ziploc baggies. Students see how mass is unchanged in a closed system.