If this picture doesn't tell you enough how awesome this experiment was, you will definitely have to try it with your kids.
Simple supplies - empty bottles, vinegar, baking soda, balloons and a funnel.
Fill the balloon with baking soda using the funnel.
Put just a little bit of vinegar in the bottle. Maybe half an inch or so.
Put the end of the balloon around the mouth of the bottle and let the baking soda drop into the vinegar.
The balloon will blow up-up-up.
Note this - for some reason when I have seen this one floating around on Pinterest, the comments claim it is a replacement for helium balloons. This is so NOT the case, so don't expect that.