Digging box

My little girl loves to dig. However after her attempt to reach Australia, we have had to put chicken wire on the bottom but she still likes to dig in litter trays and on rugs and sofas... Was thinking of using our old recycling box and filling in so she can dig in it but not sure what to put in it - should I just go for soil or something cleaner so I can put it in the house?
Sugestions please:wave:

I use a mixture of soil and play sand in mine. I always try to keep it a bit damp so it isn't dusty when they dig it. Definately don't use builders sand :wave:

If it's in the house why not shredded paper or newspapers?