I'm sorry I haven't responded sooner. I've been looking into possibilities between migraines.
My first recommendation would be the crayola stuff above. It's easy to find and I've read several threads here in which others have successfully used it.!
My second thought was paper clay. I've known several people who loved it so much it became their clay of choice. I haven't used it yet, but I found an interesting recipe with exact weights and a video showing how to make it.
http://www.ultimatepapermache.com/new-air-dry-clay-recipe
One note of caution: check the ingredients of the joint compound if you try this one. I know fungicides can be harmful to snakes. Hopefully there are types of joint compound that are snake-safe. I'm betting there's info on that in the DIY section...just can't remember off the top of my head. So this recipe needs a little more research prior to trying it out. Sorry about that.
I'm tempted to try something like this once we have enough junk mail again. My husband likes to throw that stuff in a bunch of water, but I have yet to convince him to make homemade paper or do something equally creative with the remnants yet.
This last type is something I happened upon purely by accident. I'm excited to try this one myself!
http://m.instructables.com/id/How-to-make-Cold-Porcelain-Clay-air-dry-modelling/
And an updated recipe:
http://m.instructables.com/id/Cold-Porcelain-Clay-Beads/
And one more recipe, might be more difficult to find the ingredients and make, but "unbreakable" sure sounds great:
http://www.clay-it-now.com/coldporcelainrecipe.html
They all take time to fully dry, and in my experience, letting a piece dry slower leads to less stress overall and less cracking. You don't want it to take so long to dry that it grows mold, though. There's a fine line between long and too-long.
I also have an idea for using old cardboard. The Starbucks sleeves on cold drinks whose sides have sweat gave me an idea for making a twisty, gnarled old tree in the 20-long viv I'm working on now. I might use some glue or papier-mache medium (flour and water) in that project, depending on how it feels when I start messing around with it. In theory, I should be able to soak cardboard until the fibers are pliable, workable, and may not need anything more than water until I seal it.
...Which reminds me that I haven't looked into sealants yet. That research comes next. Acrylic paint is easy to find, so I'd go with a non-toxic acrylic for paint.
I hope this helps give you some other options, if you're still interested. Good luck, and have fun playing with it! You learn through the process, so even if you lose some work, you've still gained experience. I'll try to post again once I have some info on sealing pieces.