To begin, who doesn't love a terrarium? Here's one with a little twist - inside each EcoSphere are active micro-organisms, small shrimp, algae, and bacteria all acting as a self-sustaining ecosystem.
Or how about Gallium Metal. This silvery metal with a melting point of 85 degrees - so it literally melts in your hand! Let it melt in your hand, then pour it into a dish or tray and it freezes back into a solid. Crazy!
If something more practical suits you, check out this Klein Bottle - perfect for any mathematician. A real Klein bottle is an open object with no inside and no outside (which can only exist in four dimensions) but this glass bottle in the third dimension can hold liquid... Interested? Read about the math here.