I'm digging all the "sensory play" ideas on Pinterest, and Jack loves them too.
This is a pan of old lentils, rice, and beans. He drives his trucks in them, practicing digging and loading his dump trucks. I hid some pretend coins in there, so he can dig for buried treasure over and over. We also shake the pan flat and practice writing letters and shapes with our fingers. Easy clean up with a vacuum or broom.
This pan of baking soda idea came from Trish. She said sometimes she even packs a dinosaur in a water and baking soda lump, and lets Wolfie hatch the egg by dripping the vinegar on it. I didn't go that far, but still Jack LOVED this. First making the colored foam. Then digging around with trucks and dinosaurs. Then adding more and more vinegar until it was more swamp than sand.
Today I had Wolfie over for babysitting. These two were being pretty turdy, so I had to whip out home-made slime. Also called, home-made Gak, it's a simple recipe. Just water, elmer's glue, and borax. Luckily I had just enough glue to make a batch. There are many recipes online. At first they were timid to get messy. But after a few minutes they embraced the slime and were coming up with several ways to play it. It was even Wolfie's idea to add dinosaurs. This kept them busy for 45 minutes after lunch, which is, like, eternity in toddlerhood.
If anyone has more, cheap, indoor play ideas, I'm all ears.