Substrates:
"The packaging of many substrates claim that they are completely digestible.
However, please be very wary of claims from the manufacturers of these products!
Unlike products for human consumption or use, the safety of products used
for our pets is under no such regulation. There are few truly safe substrates."
(quoted from..) But, I believe some of the safer ones would be:
Newspaper: It doesn't look great but it's cheap and
easy to replace. This is good for ill dragons that need to have a really clean
environment. Crickets do tend to hide under the sheets though.
Tile or Vinyl: It's easy to clean and replace, and very
safe, I just can't imagine it's much fun for the lizard to walk on. If you
use this at least give them good branches to climb on.
Lizard Carpet: if you can't find sheets large enough for
your cage, you can always hot glue them together with another sheet of fabric
on their underside. This allows you to wash the carpet: because it can still
expand and contract while remaining flexible, and helps keep it from getting
all messed up and pushed around in the cage. You need to keep your dragons
nails trimmed if you use this or they'll often get snagged in the fibers.

Play Sand: because of it's round particle size, and
due to the fact that it doesn't clump as badly as most sands, it's usually
an alright choice for healthy adult dragons. Just be sure to strain it through
some fine mesh, like the screen from a window, or a kitchen strainer. The
other good thing about it: it's pretty darn cheap. You can pick it up at some
big toy stores and hardware stores like Lowes.

Organic Millet? I've heard people use this but I've
never seen it. It certainly sounds good when described. It doesn't clump,
it passes easily if ingested, and it only gets moldy if it gets wet or it's
too humid- which isn't much of a problem in a proper bearded dragon enclosure.
(More info on this: Kricket's
Kritters)
Decor: Rocks, Branches, Caves, Coming Later...

