lightweight "clay"?

[marajane01.livejournal.com]

I'm going to be working on a project in which I will need to use some form of air-drying clay, modeling agent, whatever. I also need it to be lightweight, so I though of Crayola Model Magic and Fimo Air-Drying Clay- have any of you used either of these before? When they dry, are they hard or do they stay kind of soft and springy? Thanks VERY much!