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Gravel or a gravel-sand mix (cheaper, arguably better) are fine run footings. They will help keep the run dry, thus less smelly/fly-ey, too.
If you use a gravel-sand mix, or relatively small gravel, that will be plenty soft and scratch-around-in-able. You can always toss in weeds you yanked out of the garden (NOT toxic ones, obviously, but most are fine) for them to nibble and poke at. Whether to mix in other organic matter, like dead leaves, is a personal choice but remember that they will break down and make the run muddier and wetter.
Good luck and have fun,
Pat