Indoor sand box

So my son got a sand box for his birthday a few months back. We were doing great with play sand and it really wasn’t an issue. But I was stupid and had walked away to go to the bathroom today and left the top off which I don’t do ever because I have cats. Well one of my cats decided to pee in the sand box. I cleaned it up and got rid of the sand in the box and cleaned up the box itself with some safe chemicals. Now I’m just wondering what I could use in substitute for sand because my son LOVED the sand box. It’s only indoors because we do not have a back yard at our apartment.

My son is three if anyone is wondering about age appropriate ideas.

Thank you!