You can use baking soda and vinegar to blow up a balloon. You put vinegar in a bottle. Place some baking soda in a deflated balloon by using a funnel to get it into the bulb part of the orb. With the top of the balloon hanging to the side, connect the neck of the balloon to the bottle holding the vinegar. Once you have connected them and have a good seal around the bottle, bring the balloon upright shaking the baking soda into the bottle. The chemical reaction will cause the balloon to fill with carbon dioxide as it inflates.
In order to approve your balloon is full of carbon dioxide, you can test the air in the balloon. After it inflates, seal the bottom of the balloon. Place a lit candle in a jar and then slowly release the air from your balloon into the jar. As the carbon dioxide fills the space it will force the lighter air out. The candle will eventually go out not because of the wind action, as you are not letting the air out that quickly, but because of the carbon dioxide choking the flame. Without the oxygen the flame cannot continue to burn.
To showcase what density is and how denser things sink, you can use baking soda and vinegar with regular bubbles. Get a large bowl and put about a quarter cup of baking soda in the bottom. Slowly add three-quarters of a cup of vinegar and let the mixture bubble and burst. This combination is releasing carbon dioxide. Then, using your regular bubbles, blow them into the bowl. These bubbles will float above the mixture, because the carbon dioxide is heavier than the air inside the bubbles so they will seem to hover in the bowl.
You can create magic beans and rice with baking soda. Use a wide mouthed vase and fill it with water. You can add food coloring if you would like to make the experiment more colorful. Then drop in one-quarter cup of vinegar and three teaspoons of baking soda. The bubbles will begin to form in the water. Then you can slowly add some rice, beans, even pasta to watch it react to the bubbles. The experimenters can guess which items will rise and fall more quickly and record the outcomes.