Cornstarch is a good one to add, because the texture when mixed with water is so interesting. Also it looks a lot like baking soda, so it's interesting to note that only one makes the reaction with vinegar. Salt, for texture, and because it dissolves in liquids. Dish soap is a fun addition. Spices, for scent and color. I make a tray with lots of different ingredients, empty (small!) containers for mixing (small, or all your ingredients are gone in seconds), spoons or sticks for stirring, pipettes for transferring liquids. Fun for ages!

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