It costs you nothing - just some rocks from your yard, a fair deal of manipulating and practice, but once rock stacks are created, the satisfaction is immense. Forget sand and sand rakes to relax, this is becoming the new zen hobby.
The patience required to stack stones means being wholly present in the moment. It also means surrendering to nature and physics without cursing or becoming angry, but being thankful when it finds its ideal balance and you create what you thought was impossible.
If that isn't the meaning of meditation, I don't know what is!