Some simple, if possibly messy, fun. Ingredients: Water and corn starch, baking powder and vinegar.
The baking powder and vinegar mix to release carbon dioxide gas. If you put it inside something with a tight cap that can blow off, you've got a 'rocket'. Just be sure to aim it away. I don't really remember well, but I think equal vinegar and baking powder is the right recipe. But it's something to experiment with.
Corn starch and water is a chance to explore the mechanical properties of matter. (read: mess around while claiming to be doing science) Ordinary fluids, like air or water, react straightforwardly to pushing on them. If you push, they move out of the way. Push harder, they move out of the way faster. Corn starch and water (again, I think it's equal amounts, but experiment) are a different kind of thing. Set a marble on top of the mixture and it will sink through. Throw the marble at it, and it will bounce. !? Experiment. It makes a difference how fast you push. Lots of room for experimentation.
Anyone else have comparably simple experiments?