Everyone who knows me has remarked on my indoor sandbox at least once.
If you don’t have the stomach for sand in the house, get a 20 lb bag of rice and use it to fill a sand table or under-bed storage box. Vacuum after playtime is over, or get a small brush and dustpan the kids can use, Montessori style, to return spilled rice to the box.
If you’re lucky enough to have snow but not lucky enough to have weather nice enough for outdoor play, put a tarp under the box and fill it with snow. Let the kids wear mittens and gloves, make hot cocoa, and enjoy your indoor “snow day!”