I saw this at the Bay Area Discovery Museum -- it packs like wet sand but never dries out! My kids loved playing with it and I had to drag them out of the room.

There are several different options, including just the sand or a kit with the sand, pan, and sculpting tools. It's a great option especially for families in apartment buildings or other places with no access to a yard or sandbox.

Got to love a tactile product that does not dry out.