One of my most successful activities (if you are measuring success by the kids' reactions) was one I called Singing Time Science. I have seen variations of it on several different blogs. I was originally going to call it Singing Time Magic, but I happened to find a pair of safety goggles in the primary room as I was setting up so I just changed the name and had the child doing the "experiment" wear the safety goggles.
I had 5 large clear plastic cups filled with equal parts water and vinegar. Then I had 5 white plastic spoons and put a drop of food coloring in each spoon and covered it with a big pile of baking soda. When the child took the spoon and mixed it in the cup, it fizzed like crazy and turned the liquid to whatever color was in the spoon. I had 5 pieces of construction paper up on the board that corresponded to the different colors of food coloring. As each cup was mixed, we would turn over that color of paper and sing the song. It was a BIG hit.
**A variation of this that I have seen and that I may use again sometime would be to have all the cups contain just water except for one. That way the water still turns colors, but only 1 fizzes up and the kids will be excited to find out which one it is.
You can see pictures of all this here: http://primarysingingtimeideas.blogspot.com/2009/02/magic-potion.html