To give the students a good idea of how different materials conduct heat at different rates, get a block of aluminum and a block of high density foam (about 10cmx10cmx1cm).  Paint them both black.  Put a number 1 on the side of one and number 2 on the side of the other - just for identification purposes when you do the demo.   (You can also buy these premade from many science supply catalogs.) 

On the day of the demonstration, pass them around the room for everyone to feel (don't tell them what they're made of yet).  The aluminum will feel much colder than the foam.  Then tell them that you will put a piece of ice on each one to see which will melt faster.  Ask students to hypothesize which will melt the ice faster.  Pretty much all of the students will think that the ice will stay solid longer on the colder-feeling block.  Then, do the demonstration.  If you have an Elmo projector, use it.  It can be hard for students in the back to see what's happening. 

Students will be surprised at how fast the ice melts on the "cold" block.  You can then have them try to figure out what happened, what the blocks are made of, and discuss conduction and specific heat with them.  Also, if you have a second set of blocks, you can show them that even though one feels cold, they are both room temperature.