Sounds like the baked children's play sand will be the easiest option
You can actually make a large house/hide out of an upturned shoe box with a door cut out of it. Or a rightwayup shoebox with the bottom cut out and a door cut into it - then you have a lift off roof. If you wanted to try one at some point. A house can still get wee stained though, even with a potty inside - sometimes they miss.