I don't know where I first heard of this idea, but it's a good one, so I tucked it away in my brain for future use. Yesterday dawned gray and rainy, and I knew we'd be needing some fun inside activities to keep us sane.
I poured some cornmeal into a 9x13" pan and added a set of construction vehicles for Roger to play with. Simple, easy, and so much fun (and only a little messy--I put the 9x13" pan on top of a large jelly roll pan to contain the bits that escaped--you could also put down a drop cloth).
Cornmeal is great for this, because it looks like sand, but you could also use rice, dry beans or lentils, or even birdseed--whatever you happen to have on hand.