When we look for fun things to do, we like to make big messes. This particular activity fits the bill admirably!
Here is a list of what you need: 1 c. cornstarch, 1 box of baking soda, 1 1/2 c. water. Mix the dry ingredients in a medium saucepan. Stir in the water with a wooden spoon, or metal, just don't use a whisk. If you do, you'll regret it later, I can tell you from experience...big mess #1.
Now that they are all mixed up, put them on the stove on low to medium heat and prepare to stir like mad for about half an hour. Don't think you can walk away from this one and come back later. Oh no. Picture lumpy gravy and a gelatinous mass...big mess #2. It will fizzle and splatter, but don't let that deter you from stirring. Mix, mix, mix, for all you are worth. Sooner or later, it will start to coagulate. When it does, keep stirring! In fact, if your arm threatens to fall off and march away, ignore it. It probably doesn't really mean what it says.
When the mixture gets all glumpy and sticky and you can stir and see the bottom of the pan, it's approaching done. It will never roll up into a ball, like playdough; but it will become easy to move off of the side of the pan without leaving wet residue. At that point, take it off the heat and let it cool a bit so you still have fingerprints when you are done. You don't want to overcook it and let it start changing colors much or I'm pretty sure you will get big mess #3. I don't know, just sayin.'
When it is cool enough, turn it out onto a surface that has been heavily dusted with cornstarch. Just don't slam it or...big messes #4, 5 and 6 (add one for every kid that has their nose too close). Knead it and work with it until it is clay like and ceases to resemble a big pile of mashed potatoes.
At that point, shape it into anything you like, place it on a baking sheet and bake it for 30 minutes at 300 degrees Fahrenheit. When the 30 minutes are up, turn off the heat and leave it in the oven for another hour to harden off. If, for some strange reason, your creations are still damp at that point, leave them to dry on a wire rack overnight. Make sure they are dry, dry, dry, or they will end up being a science experiment rather than an artistic masterpiece.
When you are sure they are dry (see above), they can be painted...big mess #7.
Edited to add last photo, 2/17/11.