I braved my son's 3rd grade classroom this week and brought in some dry ice and liquid N₂ demos to couple with their science unit on solids, liquids, and gases.  The kids LOVED it!  And 3rd graders are such a cheap thrill after cynical college students. 

Me:  So what do you think is going to happen when I put this balloon in the liquid N₂?

Kids:  IT'S GONNA EXXXXPLOOOOOOOOOOOOODE!!!!!!! (all of them covering their ears)

Here are 2 demos that I do when I am talking about structures of metals and alloys:

 

The Atomic Trampoline Kit available from ICE:  Guaranteed wow factor!

http://mrsec.wisc.edu/Edetc/background/amorphous/

Memory Metal: I have a stash of 2" long wires so that I can give one to each student.

http://mrsec.wisc.edu/Edetc/background/memmetal/

Has anybody ever done the liquid oxygen demo?

(to prove its paramagnetic and that simple Lewis structures don't accurately describe the bonding in O2?)

I'm doing it monday.  wish me luck!