Fascinating project. The solution was right in your question; make it like a cracker — flour.
You're essentially making a sturdy paste instead of a glue. I mixed:
- 3 parts sand
- 1 part all-purpose or bread flour
- Water, just enough to wet the sand
I drained off any excess water, packed it into a mold and let it dry in the sun (1-2 hours?). I guessed at the proportions empirically, but it worked great!
The disc to the right is a failed experiment with cornstarch
The coin was pretty solid… yet breakable. I could carry it around and drop it from a few feet without breaking it, yet I could snap the disc with a bit of deliberate force, and crush it down into sand by pounding it or working it between my fingers.
…after it is crushed it should appear as close to sand as possible.
Bonus that you're not adding anything permanent or inorganic to the sand. I could not see any flour when I crushed the coins back into sand, and dropping it into a cup of water broke it down completely (which makes it easy to pour off the flour-silt, if that's important to you).