Another material I thought of was rubber. You can get big sheets of rubber with a variety of thickness and hardness pretty cheap from http://www.mcmaster.com
Here are some items not to do ...
Electro Magnet with a nail, some wire, & a 6volt battery
Solar System with styrofoam balls
Mold or pseudo Pennicillan growing on cheese ...
Plants & Photosynthesis
And absolutely never ... I repeat NEVER ...
Paper Mache' volcano with vinegar, baking soda & red food coloring eruption ...