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)

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/