I'm musing about how to give students an intuitive feeling about density by letting them lift a same sized volume of different materials, e.g. 1 liter of water, a 10x10x10 cm cube of iron, lead etc. So far, the densest material accessible and affordable to a teacher would probably be mercury (I certainly remember my chemistry teacher letting us lift a small bottle, maybe 100ml, makes for an unforgettable impression).
Do I have any chance of getting anywhere above the 13g/cm$^3$ of mercury? I sorted the elements in the Wikipedia Density article. Gold and Platinum require to win the lottery first. A liter of Plutonium needs connections to evil people, plus a lot of safety measures, so is right out :-) Osmium? Rhenium?
Are there elements or compounds denser than mercury usable for a demonstration? Are there heavy metal compounds that can be denser than the metal itself or is there a physical constraint prohibiting such a property? (I don't want to look at the far ends of a p/T phase diagram, standard temperature and pressure is okay).