Dirt

You may be familiar with they sign that says:

BOY: Noise with dirt on it.

My inlaws have a large wooden box filled with nice dark dirt.  They use it to plant tomatoes.  Where there are no tomatoes to be planted, the boys love to play in it.  I much prefer it to a "sand" box and Lewis has even agreed to build one for our house.  But ours will be strictly for play.  Not tomatoes.