I found this recipe in a science experiment book for kids. Mix one cup of cornstarch with half a cup of water, add a few drops of food coloring and you have something that'll keep your kids entertained for two episodes of American Idol, one episode of The Barefoot Contessa and one of Bobby Flay.
Once you put the three ingredients together in a mixing bowl, hand it to your kids and just let them get their hands in it. There'll be a lot of giggles and shrieks, followed by actual stretches of Q.U.I.E.T as they discover that cornstarch is sticky and gooey, but turns hard like mud when dry, which turns back to a gooey mess once it gets back into the wet mixture.
Just let them do whatever they want with it. Make Shrek hands and legs, watch the mixture drop on the floor and harden and then scoop it up like dried wax, coat a section of the floor with it and hack it up with a plastic spoon. There's no end to the wonders of cornstarch + water. A newspaper to catch all the mess is not even necessary, because cleanup is a breeze. Once the kids are done with it, it'll probably be cakey, dry and powdery, in which case you just sweep it up and toss it in the bin.
Now why didn't I discover this sooner?