This was a fun, easy activity to throw together for Quartus. And it gave him opportunity to work on fine motor skills!
Put several drops of food coloring on the bottom of a cake pan in random places. Then cover the whole pan with baking soda. Put a thick layer of it so you can't see the food coloring anymore.
Next, give your child a bowl of white vinegar and an eye dropper (along with an admonition to be very careful not to get any vinegar in their eyes.)
As he squirts vinegar on the baking soda, it will bubble up. If he squirts it over the place where food coloring is hiding, then the bubbles will be colored.
This kept Quartus entertained for quite a while. And then when he grew bored, the big kids took over and enjoyed it too!