I love this: Sandy Candy, a.k.a. candy sand that you can pour into pretty patterns.
Of course, you could do the same thing with unmade Jell-o or colored sugar, probably much cheaper, but hey.
There's also a candy pebble version of the candy that can also be layered. As a kid, I had a pencil from the Science Museum of Minnesota that had (real, not candy) tiny stones in it, and I used to always take out the eraser and dump the smooth little stones out and play with them. Of course, not all of them ever found their way back inside.